cros_ec: use uint instead of uint8_t for cmd param

Chromium EC commands can be up to 16-bits, so using a uint8_t here can
cause truncation. Update to use a uint instead.

It looks like this should likely have been done as a part of
9fea76f5, but this function was skipped
for some reason.

Signed-off-by: Michael Auchter <michael.auchter@ni.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>