V4L/DVB: Video : pwc : Fix regression in pwc_set_shutter_speed caused by bad 	constant => sizeof conversion.

Regression was caused by my commit 6b35ca0d
which determined message size using sizeof rather than hardcoded constants.

Unfortunately pwc_set_shutter_speed reuses a 2 byte buffer for a one byte
message too so the sizeof was bogus in this case.

All other uses of sizeof checked and are ok.
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: stable@kernel.org
Signed-off-by: Martin Fuzzey <mfuzzey@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>