greybus: uart: fix incomplete receive-data sanity checks

Fix incomplete receive-data sanity checks.

The payload size was never verified before parsing the uart header and
neither was the uart-header data size verified against the actual
payload size, something which could lead to information leaks when
passing data beyond the payload buffer to the tty layer.

Also remove the incorrect check against the maximum (tx-buffer) payload
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>