Fix SerialPIO sampling, avoid reading random garbage (#544)

Adjust the 1/2 bit time to match the number of extra cycles in the actual
PIO loop.

Throw out the entire start bit, which results in sampling the data at the
midpoint and not the starting time of a bit (which was causing random
failures on read data).

Tested at 300bps all the way to 2,000,000bps using a loopback connection.

Fixes #360

cores/rp2040/SerialPIO.cpp

@@ -97,7 +97,7 @@ void __not_in_flash_func(SerialPIO::_handleIRQ)() {
     }
     while (!pio_sm_is_rx_fifo_empty(_rxPIO, _rxSM)) {
         uint32_t decode = _rxPIO->rxf[_rxSM];
-        decode >>= 32 - _rxBits;
+        decode >>= 33 - _rxBits;
         uint32_t val = 0;
         for (int b = 0; b < _bits + 1; b++) {
             val |= (decode & (1 << (b * 2))) ? 1 << b : 0;
@@ -229,7 +229,7 @@ void SerialPIO::begin(unsigned long baud, uint16_t config) {
         pio_sm_clear_fifos(_rxPIO, _rxSM); // Remove any existing data
 
         // Put phase divider into OSR w/o using add'l program memory
-        pio_sm_put_blocking(_rxPIO, _rxSM, clock_get_hz(clk_sys) / (_baud * 2) - 2);
+        pio_sm_put_blocking(_rxPIO, _rxSM, clock_get_hz(clk_sys) / (_baud * 2) - 5 /* insns in PIO halfbit loop */);
         pio_sm_exec(_rxPIO, _rxSM, pio_encode_pull(false, false));
 
         // Join the TX FIFO to the RX one now that we don't need it