cfg80211: fix incorrect assumption on last_request for 11d

The incorrect assumption is the last regulatory request
(last_request) is always a country IE when processing
country IEs. Although this is true 99% of the time the
first time this happens this could not be true.

This fixes an oops in the branch check for the last_request
when accessing drv_last_ie. The access was done under the
assumption the struct won't be null.

Note to stable: to port to 29 replace as follows, only 29 has
country IE code:

s|NL80211_REGDOM_SET_BY_COUNTRY_IE|REGDOM_SET_BY_COUNTRY_IE

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>