[media] em28xx: input: fix oops on device removal

When em28xx_ir_init() fails due to an configuration error, it frees the memory
of struct em28xx_IR *ir, but doesn't set the corresponding pointer in the
device struct to NULL.
On device removal, em28xx_ir_fini() gets called, which then calls
rc_unregister_device() with a pointer to freed memory.
Fixes bug 26572 (http://bugzilla.kernel.org/show_bug.cgi?id=26572

)

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>