V4L/DVB (5019): Fix the frame->grabstate update in read() entry point.

The Coverity checker spotted that in usbvision_v4l2_read(), the variable
"frmx" is never assigned any value different from -1, but it's used an 
an array index in "usbvision->frame[frmx]".
Thanks to Adrian Bunk <bunk@stusta.de> for warning about that.

Signed-off-by: Thierry MERLE <thierry.merle@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>