brcmfmac: fix out of bound access on clearing wowl wake indicator

Clearing the wowl wakeindicator happens with a rather odd
construction where the string "clear" is used to set the iovar
wowl_wakeind. This was implemented incorrectly as it caused an
out of bound access. Use an intermediate variable of correct
length and copy string in that. Problem was found using coverity.

Reviewed-by: Arend Van Spriel <arend.vanspriel@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Signed-off-by: Hante Meuleman <hante.meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>