wl18xx: fix last tx rate calculation

The last tx rate calculation didn't take into account
the different indices of 11a and 11g rates tables.

Add the required alignment (count only from the first
11a rate in case of 11a)

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>