drivers: serial: sam u(s)art: correct interpretation of RXRDY flag

The receiver ready flag of the (channel) status register
for the sam controller was not being interpreted correctly
for both the uart and usart implementation,
according to the uart api.
Tested and confirmed using the sam4sa16ca.

Signed-off-by: Jaro Van Landschoot <jaro.vanlandschoot@basalte.be>