brcmfmac: fix sparse warnings in e-scan related code

With the introduction of e-scan mechanism in brcmfmac
(e756af5b brcmfmac: add e-scan support.) a couple of sparse
warnings were introduced. This patch resolves those.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>