mwifiex: use correct firmware command to get power limits

"priv->max_tx_power_level" and "priv->min_tx_power_level" variables
are initialized to maximum and minimum power levels supported by
hardware by sending correct firmware command.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>