wl1271: Allocate TX descriptors more efficiently

On each TX descriptor allocation, a free entry is found by traversing the TX
descriptors array.

Improve this by holding a bitmap of all TX descriptors, and using efficient
bit operations to search for free entries.

Signed-off-by: Ido Yariv <ido@wizery.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>