[media] gsc-m2m: Remove an unused var.

drivers/media/platform/exynos-gsc/gsc-m2m.c: In function 'gsc_m2m_reqbufs':
drivers/media/platform/exynos-gsc/gsc-m2m.c:365:20: warning: variable 'frame' s
et but not used [-Wunused-but-set-variable]
  struct gsc_frame *frame;
                    ^

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>