mwifiex: optimize driver initialization code

1) removal of unnecessary mwifiex_device structure
2) avoid passing adapter pointer to mwifiex_init_sw()
3) remove local variable drv_mode_info in mwifiex_add_card()
4) type change in mwifiex_bss_attr to match mwifiex_private
5) removal of more wordy comments

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>