brcmsmac: request firmware in .start() callback

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 <[email protected]>
Reviewed-by: Piotr Haber <[email protected]>
Reviewed-by: Hante Meuleman <[email protected]>
Signed-off-by: Arend van Spriel <[email protected]>
Signed-off-by: John W. Linville <[email protected]>