[media] em28xx: Fix em28xx deplock

When em28xx extensions are loaded/removed, there are two locks:

a single static em28xx_devlist_mutex that registers each extension
and the struct em28xx dev->lock.

When extensions are registered, em28xx_devlist_mutex is taken first,
and then dev->lock.

Be sure that, when extensions are being removed, the same order
will be used.

Reviewed-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>