wifi: iwlegacy: add missing mutex protection in il3945_store_measurement()

wifi: iwlegacy: add missing mutex protection in il3945_store_measurement()

il3945_store_measurement() calls il3945_get_measurement() which internally
calls il_send_cmd_sync() without holding il->mutex. However,
il_send_cmd_sync() has lockdep_assert_held(&il->mutex) indicating that
callers must hold this lock.

Other sysfs store functions in the same file properly acquire the mutex:
- il3945_store_flags() acquires mutex at 3945-mac.c:3110
- il3945_store_filter_flags() acquires mutex at 3945-mac.c:3144

Add mutex_lock()/mutex_unlock() around the il3945_get_measurement() call
in the sysfs store function to fix the missing lock protection.

Signed-off-by: Ziyi Guo <n7l8m4@u.northwestern.edu>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Link: https://patch.msgid.link/20260125193005.1090429-1-n7l8m4@u.northwestern.edu
Signed-off-by: Johannes Berg <johannes.berg@intel.com>