net: wirelesse: wcn36xx: pull allocation outside of critical section

Commit [3469adb3: fix potential NULL pointer dereference] introduced
a check of msg_ind allocation, but omitted allocation of msg_ind->msg.
Moreover, it introduced two if statements, which looked a bit clunky.

This commit moves allocation code outside of the critical section so
there's no need to dance around mutex_unlock, and adds the missing
allocation check.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>