Fix rtc6715 L2 and L3 frequencies

[Lignar]

L2 and L3 channels of the built-in analog module were incorrectly tuned to 5383MHz and 5420MHz respectively.
This PR updates L2 to 5399MHz and L3 to 5436MHz, in line with values most commonly used on VTXs supporting analog Low Raceband, like Rush, TBS, Geprc, AKK etc.

[ligenxxxx]

I'm confused; someone suggested last time that I modify the frequencies of L2 and L3.
It seems there are two different standards circulating.
#535

[Lignar]

Oh, I didn't notice that. After some digging through VTX tables, here's the variants I've found:

5362 5399 5436 5473 5510 5547 5584 5621 (my version): Rush (Max Solo, Tank Solo, Dual-Band), TBS Unify PRO32 DP, GEPRC (Rad Mini, Maten, Maten Pro, HGLRC High Power, AKK (TX5000AC, TX8000AC), ReadyToSky Ultra, SpeedyBee TX Ultra, Skyzone (Atomrc TX1201, TX2500, TX2501, Axisflying Terk, Pilotix, Bayckrc Starfish

5333 5373 5413 5453 5493 5533 5573 5613: Foxeer Reaper (Extreme, Infinity), AKK (Ultra Long Range AC, TX5000AC, TX8000AC, Alpha), Peak

Honestly, I don't know where 5383 and 5420 (and old 5350 for L1) came from. Do you happen to remember what VTX was that person using?

(And yes, AKK is using two different sets of L-band frequencies in their official VTX tables, depending on model, with some supporting both as different bands...)

I tested my values with TBS Unify PRO32 DP - using the values from current main, L2 and L3 had heavily distorted video. With changes from this PR video on those channels worked well

[SentinelFour]

I think as long as we follow the TBS and BetaFPV frequencies, we should be good.

vtxtable band 6 RACE_LOW L FACTORY 5362 5399 5436 5473 5510 5547 5584 5621
https://www.team-blacksheep.com/media/files/vtx-table-for-betaflight.txt

vtxtable band 6 LOWRACE L FACTORY 5362 5399 5436 5473 5510 5547 5584 5621
(taken from a random Air 65 CLI from their homepage)

There are some deviations by other manufacturers but from my perspective, these are the frequencies to be followed. Last time I checked, these were also the frequencies the HDZero VTX would use when low band has been unlocked. But I would need to double check this when I am at home.