wl1251: allocate space for firmware with vmalloc()

Earlier firmware was stored to a memory area allocated with kmalloc()
but finding a a contiguous area of memory long enough for the firmware is
very difficult in certain cases. better to allocate the memory for firmware
with vmalloc() instead and use a small buffer for DMA transfers.

Thanks to Eero Tamminen for the idea.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Reviewed-by: Vidhya Govindan <vidhya.govindan@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>