greybus: uart: Fix sparse warning

Greg reported sparse picked up the following warning:
/home/gregkh/ara/greybus/uart.c:105:34: warning: cast to restricted __le16

This is due to the control variable in gb_uart_serial_state_request which
needs to be declared __le16 not __u16.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reported-by: Greg Kroah-Hartman <gregkh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>