MIPS: ralink: enable PCI support only if driver for mt7621 SoC is selected

MIPS: ralink: enable PCI support only if driver for mt7621 SoC is selected

[ Upstream commit 3b2fa0c92686562ac0b8cf00c0326a45814f8e18 ]

Some versions of SoC MT7621 have three PCI express hosts. Some boards
make use of those PCI through the staging driver mt7621-pci. Recently
PCI support has been removed from MT7621 Soc kernel configuration due
to a build error. This makes imposible to compile staging driver and
produces a regression for gnubee based boards. Enable support for PCI
again but enable it only if staging mt7621-pci driver is selected.

Fixes: c4d48cf5e2f0 ("MIPS: ralink: deactivate PCI support for SOC_MT7621")
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: ralf@linux-mips.org
Cc: jhogan@kernel.org
Cc: john@phrozen.org
Cc: NeilBrown <neil@brown.name>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20191019081233.7337-1-sergio.paracuellos@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>