Revert "ssb: Prevent build of PCI host features in module"

This reverts commit 882164a4.

Above commit added "SSB = y" dependency to the wrong symbol
SSB_DRIVER_PCICORE_POSSIBLE and prevented SSB_DRIVER_PCICORE from being
selected when needed. PCI core driver for core running in clienthost
mode is important for bus initialization. It's perfectly valid scenario
to have ssb built as module and use it with buses on PCI cards.

This fixes regression that affected all *module* users with PCI cards.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1572349


Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>