iwlwifi: fix power index handling

The power index that the user gives as module parameter
in in range 1-5, but we need to decrease it in order
to create an array index out of it (0-4) for the power
table command.

Signed-off-by: Amit Beka <amit.beka@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>