staging: rtl8723bs: rtw_get_sec_ie(): Change return type

Remove assignment of return value of function rtw_get_sec_ie as it is
never checked or used.
Remove return statement of rtw_get_sec_ie as its return value is never
stored or checked or used.
Change return type of rtw_get_sec_ie to void.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>