media: vim2m: don't use curr_ctx->dev before checking

It seems that it is possible that dev to be null, as there's
a warning printing:
	"Instance released before the end of transaction"

Solves this warning:
	drivers/media/platform/vim2m.c: drivers/media/platform/vim2m.c:525 device_work() warn: variable dereferenced before check 'curr_ctx' (see line 523)

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