greybus: uart: add max-payload sanity check

Let's be well behaved and add a sanity check on the maximum greybus
payload size to avoid underflow on the calculated buffer size.

Reviewed-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>