tty/serial: atmel: Add is_half_duplex helper

Use a helper function to check that a port needs to use half duplex
communication, replacing several occurrences of multi-line bit checking.

Fixes: b389f173 ("tty/serial: atmel: RS485 half duplex w/DMA: enable RX after TX is done")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Razvan Stefanescu <razvan.stefanescu@microchip.com>
Acked-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>