uvcvideo: Fix data type for pan/tilt control

The pan/tilt absolute control value is signed value. If minimum value
is minus, It will be changed to plus by clamp_t() as commit 64ae9958a62.
([media] uvcvideo: Fix control value clamping for unsigned integer controls).
It leads to wrong setting of the control values. For example,
when min and max are -36000 and 36000, the setting value between of this range
is always 36000. So, its data type should be changed to signed.

Signed-off-by: Chanho Min <chanho.min@lge.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>