mwifiex: use firmware API revision from GET_HW_SPEC response

Newer firmware returns API revision in GET_HW_SPEC command
response. We will make use of it instead of parsing this
information from FW release number.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>