drivers: uart: atmel_sam: Added reset after error check

Following the sam4s datasheet, the OVRE, PARE and FRAME flags should be
cleared after a uart error occured. This is done writing a 1 to the
RSTSTA bit in the UART_CR.

Signed-off-by: Ibe Van de Veire <ibe.vandeveire@basalte.be>