mwifiex: remove MWIFIEX_BSS_MODE_ macros

replace them with NL80211_IFTYPE_ macros

Also remove redundant functions mwifiex_drv_get_mode()
and mwifiex_bss_ioctl_mode().

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>