mwifiex: replace u16 with __le16 in struct mwifiex_types_power_group

__le16 to u16 conversion is missing for "pg_tlv_hdr->length"
in mwifiex_get_power_level(). This creates a problem on big
endian machines.
It is resolved by changing definition of the structure
and making required endianness changes.

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>