greybus: uart: Update UART to reflect field size changes

The greybus UART protocol specification was updated to reduce the size of
the control field in serial-state-request and line-state-request. This
patch updates the kernel protocol driver to reflect the specification
changes. Once applied gbsim changes will be also be updated automatically
since gbsim depends on the header being modified directly.

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