ACS: Be more consistent with iface->current_mode checks

ACS: Be more consistent with iface->current_mode checks

Offloading of ACS to the driver changed the design a bit in a way that
iface->current_mode could actually be NULL when the offloaded ACS
mechanism supports band selection in addition to channel selection. This
resulted in a combination that is too complex for static analyzers to
notice. While acs_init() can be called with iface->current_mode == NULL
that is only in the case where WPA_DRIVER_FLAGS_ACS_OFFLOAD is in use.
In other words, the actual ACS functions like acs_cleanup() that would
dereference iface->current_mode are not used in such a case.

Get rid of static analyzer warnings by explicitly checking
iface->current_mode in acs_init() for the case where ACS offloading is
not used.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>