[media] gspca_kinect: fix kinect_read() error path

The error checking code relative to the invocations of kinect_read()
does not return the actual return code of the function just called, it
returns "res" which still contains the value of the last invocation of
a previous kinect_write().

Return the proper value, and while at it also report with -EREMOTEIO the
case of a partial transfer.

Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>