xhci: add the meaningful IRQ description if it is empty

When some xHCI host controllers fall back to use the legacy IRQ,
the member irq_descr of the usb_hcd structure will be empty. This
leads to the empty string of the xHCI host controller in
/proc/interrupts. Here is the example (The irq 19 is the xHCI host
controller):

           CPU0
  0:         91		IO-APIC-edge      	timer
  8:          1         IO-APIC-edge      	rtc0
  9:       7191         IO-APIC-fasteoi   	acpi
 18:        104       	IR-IO-APIC-fasteoi 	ehci_hcd:usb1, ehci_hcd:usb2
 19:        473     	IR-IO-APIC-fasteoi

After applying the patch, the name of the registered xHCI host
controller can be displayed correctly. Here is the example:

           CPU0
  0:         91		IO-APIC-edge      	timer
  8:          1         IO-APIC-edge      	rtc0
  9:       7191         IO-APIC-fasteoi   	acpi
 18:        104       	IR-IO-APIC-fasteoi 	ehci_hcd:usb1, ehci_hcd:usb2
 19:        473     	IR-IO-APIC-fasteoi	xhci_hcd:usb3

Tested on v3.14-rc4.

Signed-off-by: Adrian Huang <ahuang12@lenovo.com>
Reviewed-by: Nagananda Chumbalkar <nchumbalkar@lenovo.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>