mac80211: fix kconfig recursive dependency warning

Make brcmfmac depend on !TARGET_uml.
Technically, brcmfmac could be built for uml because only SDIO support
won't work on that target. However, selectively avoiding the dependency
propagation of !TARGET_uml from kmod-mmc to avoid including a reference
to BRCMFMAC_SDIO doesn't work.

In practice, brcmfmac is completely useless on uml, so let's just
disable it there.

Signed-off-by: Felix Fietkau <nbd@nbd.name>