mwifiex: remove unnecessary _set_auth functions

mwifiex_set_encrypt_mode()
mwifiex_set_auth_mode()
mwifiex_set_auth()
These functions are confusing and misleading.
And they are really not needed at all.

Some unused definitions are also removed.

Signed-off-by: Marc Yang <yangyang@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>