usb: chipidea: error on overflow for port_test_write

The write value is 8bit, but currently writing a larger number
(eg a doubled digit) is not errored but instead gets cast and
sets off an action probably undesired.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
(Change the style of commit log to fix checkpatch.pl warning)