serial: atmel_serial: include circ_buf.h

atmel_uart_port::rx_ring is defined as struct circ_buf, but circ_buf.h
is not included explicitly in atmel_serial.c. It is included only
implicitly via serial_core.h. Fix this as serial_core.h might not
include that header in the future.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>

Cc: Richard Genoud <richard.genoud@gmail.com>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Acked-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Link: https://lore.kernel.org/r/20220124071430.14907-3-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>