wl1271: Use vmalloc to allocate memory for firmware

Use vmalloc to allocate memory for the firmware image, and use a smaller
linear buffer for the actual transfer of the firmware to the chipset.

This patch is an adaptation of a similar patch for wl1251 by Kalle Valo.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>