[media] exynos-gsc: fix variable type in gsc_m2m_device_run()

In gsc_m2m_device_run(), variable "ret" is accepting signed integer
values. But currently it is defined as u32. This patch will modify
the type of "ret" variable to "int".

Signed-off-by: Shaik Ameer Basha <shaik.ameer@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>