staging: rtl8723bs: reduce stack usage of rtw_cfg80211_unlink_bss

staging: rtl8723bs: reduce stack usage of rtw_cfg80211_unlink_bss

The build of xtensa allmodconfig gives warning of:
In function 'rtw_cfg80211_unlink_bss':
warning: the frame size of 1136 bytes is larger than 1024 bytes

Instead of having 'select_network' structure as a variable use it as a
pointer.

Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20191018104837.23246-1-sudipm.mukherjee@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>