USB: xhci - fix math in xhci_get_endpoint_interval()

When parsing exponent-expressed intervals we subtract 1 from the
value and then expect it to match with original + 1, which is
highly unlikely, and we end with frequent spew:

	usb 3-4: ep 0x83 - rounding interval to 512 microframes

Also, parsing interval for fullspeed isochronous endpoints was
incorrect - according to USB spec they use exponent-based
intervals (but xHCI spec claims frame-based intervals). I trust
USB spec more, especially since USB core agrees with it.

This should be queued for stable kernels back to 2.6.31.

Reviewed-by: Micah Elizabeth Scott <micah@vmware.com>
Signed-off-by: Dmitry Torokhov <dtor@vmware.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@kernel.org