Some firmware leaves the Interrupt Disable bit set even if the device uses
INTx interrupts.  Clear Interrupt Disable so we get those interrupts.

Based on the report mentioned below, if the user selects the "EHCI only"
option in the Intel Baytrail BIOS, the EHCI device is handed off to the OS
with the PCI_COMMAND_INTX_DISABLE bit set.

Link: http://lkml.kernel.org/r/20140114181721.GC12126@xanatos
Link: https://bugzilla.kernel.org/show_bug.cgi?id=70601Reported-by: NChris Cheng <chris.cheng@atrustcorp.com>
Reported-and-tested-by: NJamie Chen <jamie.chen@intel.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
CC: stable@vger.kernel.org
CC: Sarah Sharp <sarah.a.sharp@linux.intel.com>