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: NIdo Yariv <ido@wizery.com>
Reviewed-by: NJuuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: NLuciano Coelho <luciano.coelho@nokia.com>