[media] gsc-m2m: Use an unsigned data type for a variable

The data type "int" was used by the variable "ret" in the
gsc_m2m_poll() function despite of the aspect that the type "unsigned int"
will usually be needed for the return value from a call of the
v4l2_m2m_poll() function.
Improve this implementation detail by addition of the type modifier then.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>