tty: atmel_serial: define macro for RX size

tty: atmel_serial: define macro for RX size

It is repeated in the code and there is also a big warning by
ATMEL_SERIAL_RINGSIZE. So define ATMEL_SERIAL_RX_SIZE and use it
appropriatelly.

The macro uses array_size() and kmalloc_array() is switched to
kmalloc().

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Richard Genoud <richard.genoud@gmail.com>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lore.kernel.org/r/20240405060826.2521-15-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>