USB: xhci - fix interval calculation for FS isoc endpoints

Full-speed isoc endpoints specify interval in exponent based form in
frames, not microframes, so we need to adjust accordingly.

NEC xHCI host controllers will return an error code of 0x11 if a full
speed isochronous endpoint is added with the Interval field set to
something less than 3 (2^3 = 8 microframes, or one frame).  It is
impossible for a full speed device to have an interval smaller than one
frame.

This was always an issue in the xHCI driver, but commit
dfa49c4a "USB: xhci - fix math in
xhci_get_endpoint_interval()" removed the clamping of the minimum value
in the Interval field, which revealed this bug.

This needs to be backported to stable kernels back to 2.6.31.

Reported-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Dmitry Torokhov <dtor@vmware.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@kernel.org