ssb: Do not disable PCI host on non-Mips

After upgrading an old laptop to 4.15-rc9, I found that the eth0 and
wlan0 interfaces had disappeared.  It turns out that the b43 and b44
drivers require SSB_PCIHOST_POSSIBLE which depends on
PCI_DRIVERS_LEGACY, a config option that only exists on Mips.

Fixes: 58eae141 ("ssb: Disable PCI host for PCI_DRIVERS_GENERIC")
Cc: stable@vger.org
Signed-off-by: Sven Joachim <svenjoac@gmx.de>
Reviewed-by: James Hogan <jhogan@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>