mwl8k: do rx/tx ring initialisation after loading firmware

Whether the firmware we have loaded is AP or STA firmware decides
which receive descriptor format we have to use.  Therefore, move
rx/tx ring initialisation to be after firmware loading.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>