Define explicit label for wlan sysfs fwpath

avc: denied { write } for name="fwpath" dev="sysfs" ino=6863
scontext=u:r:wificond:s0 tcontext=u:object_r:sysfs_wlan_fwpath:s0
tclass=file permissive=0

Test: wificond and netd can write to this path, wifi works
Test: `runtest frameworks-wifi` passes

Bug: 29579539

Change-Id: Ia21c654b00b09b9fe3e50d564b82966c9c8e6994
(cherry picked from commit 7d13dd80)