MIPS: ralink: deactivate PCI support for SOC_MT7621

The Linux does not support PCI on the SOC_MT7621, if it is selected the
Linux build runs into a compile error. Remove HAVE_PCI from the
SOC_MT7621 SoC.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: ralf@linux-mips.org
Cc: jhogan@kernel.org
Cc: john@phrozen.org
Cc: linux-mips@vger.kernel.org