brcmfmac: fix typo in CONFIG_BRCMISCAN

The old ifdef CONFIG_BRCMFISCAN looks wrong to me and it makes more
sense when CONFIG_BRCMISCAN is used.
This patch was just compile tested by me, but not runtime tested.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>