tty/serial: atmel: use offset_in_page() macro

Use offset_in_page() macro instead of open-coding.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Acked-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>