drivers: serial: nrfx_uarte: Add workaround for FRAMETIMEOUT corner case

When reception is restarted (STARTRX after ENDRX but no STOPRX) it is
possible that FRAMETIMEOUT countdown counter will not be started by
the first received byte if byte was already being transmitted when
STARTRX was called. If that is the only byte then it is expected that
timeout will be triggered but since FRAMETIMEOUT counter is not started
there is no FRAMETIMEOUT event which has short to STOPRX. This
situation will happen in case short buffers are used (< 5 bytes)
because then short ENDRX_STARTRX is not used then.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>