brcmfmac: use vmalloc to allocate mem for the firmware

The firmware is more than 300KB big and you should not use kmalloc for
such big allocations. This allocation with kmalloc failed on my mips
based device (BCM47186).

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>