Do not compile on ATMEGA328P at frequencies below 8MHz

[caradag]

While it compiles fine at 8MHz (internal oscillator) it fails at 4, 2 and 1 MHz. I'm using ATmega 328P with minicore. The errors I get are

`/home/camilo/Documents/sketchbook/libraries/NeoSWSerial/src/NeoSWSerial.cpp: In static member function 'static void NeoSWSerial::rxISR(uint8_t)':
/home/camilo/Documents/sketchbook/libraries/NeoSWSerial/src/NeoSWSerial.cpp:308:16: error: 'TCNTX' was not declared in this scope
uint8_t t0 = TCNTX; // time of data transition (plus ISR latency)
^~~~~
/home/camilo/Documents/sketchbook/libraries/NeoSWSerial/src/NeoSWSerial.cpp:308:16: note: suggested alternative: 'TCNT0'
uint8_t t0 = TCNTX; // time of data transition (plus ISR latency)
^~~~~
TCNT0
/home/camilo/Documents/sketchbook/libraries/NeoSWSerial/src/NeoSWSerial.cpp: In member function 'bool NeoSWSerial::checkRxTime()':
/home/camilo/Documents/sketchbook/libraries/NeoSWSerial/src/NeoSWSerial.cpp:383:28: error: 'TCNTX' was not declared in this scope
uint8_t t0 = TCNTX; // now
^~~~~
/home/camilo/Documents/sketchbook/libraries/NeoSWSerial/src/NeoSWSerial.cpp:383:28: note: suggested alternative: 'TCNT0'
uint8_t t0 = TCNTX; // now
^~~~~
TCNT0
/home/camilo/Documents/sketchbook/libraries/NeoSWSerial/src/NeoSWSerial.cpp: In member function 'virtual size_t NeoSWSerial::write(uint8_t)':
/home/camilo/Documents/sketchbook/libraries/NeoSWSerial/src/NeoSWSerial.cpp:536:18: error: 'TCNTX' was not declared in this scope
uint8_t t0 = TCNTX; // start time
^~~~~
/home/camilo/Documents/sketchbook/libraries/NeoSWSerial/src/NeoSWSerial.cpp:536:18: note: suggested alternative: 'TCNT0'
uint8_t t0 = TCNTX; // start time
^~~~~
TCNT0
/home/camilo/Documents/sketchbook/libraries/NeoSWSerial/src/NeoSWSerial.cpp:561:13: error: 'PCI_FLAG_REGISTER' was not declared in this scope
if (PCI_FLAG_REGISTER & PCIbit) {
^~~~~~~~~~~~~~~~~

exit status 1

Compilation error: exit status 1`