staging: greybus: uart.c: Remove include linux/serial.h

$ make includecheck | grep staging
./drivers/staging/greybus/uart.c: linux/serial.h is included more than once.

Signed-off-by: Darryl T. Agostinelli <dagostinelli@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>