wl12xx: Modify requested number of memory blocks

Tests have shown that the requested number of memory blocks is
sub-optimal. Slightly modify the requested number of memory blocks for
TX.

Signed-off-by: Ido Yariv <ido@wizery.com>
Reviewed-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>