cfg80211: fix NULL pointer deference in reg_device_remove()

We won't ever get here as regulatory_hint_core() can only fail
on -ENOMEM and in that case we don't initialize cfg80211 but this is
technically correct code.

This is actually good for stable, where we don't check for -ENOMEM
failure on __regulatory_hint()'s failure.

Cc: stable@kernel.org
Reported-by: Quentin Armitage <Quentin@armitage.org.uk>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>