media: exynos-gsc: fix return code if mutex was interrupted

All poll routines expect a poll flag, and not error codes. So,
instead of returning -ERESTARTSYS if the mutex got interrupted,
return EPOLERR, just like the V4L2 VB2 code.

Solves this sparce warning:
	drivers/media/platform/exynos-gsc/gsc-m2m.c:716:24: warning: incorrect type in return expression (different base types)
	drivers/media/platform/exynos-gsc/gsc-m2m.c:716:24:    expected restricted __poll_t
	drivers/media/platform/exynos-gsc/gsc-m2m.c:716:24:    got int

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>