ath6kl: Avoid null ptr dereference while printing reg domain pair

Return value of ath6kl_get_regpair() is stored in 'regpair' in
ath6kl_wmi_regdomain_event() func and it's directly accessed
in the debug prints without checking for NULL value. There are
situation to get NULL pointer as a return value from
ath6kl_get_regpair() func. Fix this.

Found this on code review.

Signed-off-by: Raja Mani <rmani@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>