brcmsmac: fix NULL pointer crash in brcms_c_regd_init()

In the function brcms_c_regd_init() the channels are validated
against the device capabilities. This is done for both 2.4G and
5G band, but there are devices that are 2.4G only, ie. BCM4313.
For that device this leads to a NULL dereference. This patch adds
a check in brcms_c_regd_init() to fix this.

Issue introduced in wireless-next tree by following commit:
cf03c5da brcm80211: smac: inform mac80211 of the X2 regulatory domain

Cc: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>