rtl8187: Fix error in setting OFDM power settings for RTL8187L

After reports of poor performance, a review of the latest vendor driver
(rtl8187_linux_26.1025.0328.2007) for RTL8187L devices was undertaken.

A difference was found in the code used to index the OFDM power tables. When
the Linux driver was changed, my unit works at a much greater range than
before. I think this fixes Bugzilla #12380 and has been tested by at least
two other users.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Tested-by: Martín Ernesto Barreyro <barreyromartin@gmail.com>
Cc: Stable <stable@kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>