brcmfmac: integrate add_keyext in add_key

brcmf_add_keyext is called when a key is configured for a specific
mac address. This function is very similar to the calling function
brcmf_add_key. Integrate this function and also use existing del_key
function in case key is to be cleared.

Reviewed-by: Arend Van Spriel <arend.van@broadcom.com>
Reviewed-by: Franky (Zhenhui) 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@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>