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: NDarryl T. Agostinelli <dagostinelli@gmail.com>
Reviewed-by: NAlex Elder <elder@linaro.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/greybus/uart.c

@@ -23,7 +23,6 @@
 #include <linux/serial.h>
 #include <linux/tty_driver.h>
 #include <linux/tty_flip.h>
-#include <linux/serial.h>
 #include <linux/idr.h>
 #include <linux/fs.h>
 #include <linux/kdev_t.h>