brcm80211: fmac: remove firmware requests from init_module syscall

As indicated in [1] on netdev mailing list drivers should not block
on the init_module() syscall. This patch defers the actual driver
registration to a workqueue so the init_module() syscall can complete
without delay.

[1] http://article.gmane.org/gmane.linux.network/217729/

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>