mwifiex: improvements in mwifiex_ret_tx_power_cfg()

1) Move common code out of switch case handling
2) Return from the function if number of bytes left in response buffer
are less than tlv size
3) Pass pg_tlv_hdr directly instead of txp_cfg to mwifiex_get_power_level()

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>