brcmfmac: split brcmf_attach() and brcmf_detach() functions

Move code allocating/freeing wiphy out of above functions. This will
allow reinitializing the driver (e.g. on some error) without allocating
a new wiphy.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>