wifi: mac80211: Avoid address calculations via out of bounds array indexing

req->n_channels must be set before req->channels[] can be used.

This patch fixes one of the issues encountered in [1].

[   83.964255] UBSAN: array-index-out-of-bounds in net/mac80211/scan.c:364:4
[   83.964258] index 0 is out of range for type 'struct ieee80211_channel *[]'
[...]
[   83.964264] Call Trace:
[   83.964267]  <TASK>
[   83.964269]  dump_stack_lvl+0x3f/0xc0
[   83.964274]  __ubsan_handle_out_of_bounds+0xec/0x110
[   83.964278]  ieee80211_prep_hw_scan+0x2db/0x4b0
[   83.964281]  __ieee80211_start_scan+0x601/0x990
[   83.964291]  nl80211_trigger_scan+0x874/0x980
[   83.964295]  genl_family_rcv_msg_doit+0xe8/0x160
[   83.964298]  genl_rcv_msg+0x240/0x270
[...]

[1] https://bugzilla.kernel.org/show_bug.cgi?id=218810

Co-authored-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Kenton Groombridge <concord@gentoo.org>
Link: https://msgid.link/20240605152218.236061-1-concord@gentoo.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>