mwifiex: change beacon parameter structure

'mwifiex_bcn_param' structure contains five parameters which
are present in beacon buffer in case of legacy scan.

'rssi' field won't be there in this buffer for extended scan.
Hence 'bssid' and 'rssi' are removed from the structure and it is
renamed as 'mwifiex_fixed_bcn_param' so that we can have common
parsing logic later for both.

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>