uart: Immediately skip reception of parity bits when not applicable

uart: Immediately skip reception of parity bits when not applicable

When the UART frame does not contain a parity bit, then immediately
advance to reception of stop bits after all data bits were received.

This eliminates the necessity to run the parity check routine when
parity does not apply in the first place. Without this change, some
"dummy" sample needs to get inspected for correct operation of the
state machine.