V4L/DVB (7550): em28xx: Fix a possible memory leak

I did notice a possible memory leak since iolock is could possibly be
called before a buffer has been freed.

This ensure s_fmt isn't called while the queue is busy thereby avoiding
iolock on already allocated buffers.

Signed-off-by: Brandon Philips <bphilips@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>