Staging: agnx: switch config_interface to bss_info_changed

Fix this build error:
drivers/staging/agnx/pci.c: In function 'agnx_config_interface':
drivers/staging/agnx/pci.c:315: error: dereferencing pointer to incomplete type
..
drivers/staging/agnx/pci.c:424: error: unknown field 'config_interface' specified in initializer

config_interface was removed in 2d0ddec5b2b
("mac80211: unify config_interface and bss_info_changed")

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>