brcmsmac: request firmware in .start() callback

The firmware is requested from user-space. To assure the request
is handled it is recommended to do the request upon IFF_UP. For
a mac80211 driver the .start() callback can be considered the
equivalent.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Piotr Haber <phaber@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>