tty: serial: atmel: Separate mode clearing between UART and USART

tty: serial: atmel: Separate mode clearing between UART and USART

When clearing the mode of the serial IP inside the atmel_set_termios()
method, make sure that the difference between the bitfields placement
of the UART IP's and USART IP's is taken into account, as some of
them overlap with each other. For example, ATMEL_UA_BRSRCCK overlaps
with ATMEL_US_NBSTOP and ATMEL_US_USCLKS overlaps with ATMEL_UA_FILTER.

Furthermore, add definitions for the Baud Rate Source Clock and the
Filter bitfields of the Mode Register of UART IP's, since they were
missing.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
Link: https://lore.kernel.org/r/20220922113347.144383-7-sergiu.moga@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>