kernel: Deactivate CONFIG_VFIO in generic kernel config

kernel: Deactivate CONFIG_VFIO in generic kernel config

Instead of deactivating this in every target config, deactivate it once
in the generic kernel config. I was asked for this config option in a
x86 64 build in OpenWrt 21.02.

Fixes: 87046e87e229 ("kernel: add missing kernel config symbol")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>