tty: serial: atmel: Use FIELD_PREP/FIELD_GET

Convert all open-coded instances of bitfields retrieval/setting
to FIELD_PREP/FIELD_GET where possible.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220922113347.144383-10-sergiu.moga@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>