USB: xHCI: Fix interrupt moderation.

Mask off the lower 16 bits of the interrupt control register, instead of
masking off the upper 16 bits.  The interrupt moderation interval field is
the lower 16 bytes, and is set to 0x4000 (1ms) by default.  The previous
code was adding 40 us to the default value, instead of setting it to 40
us.  This makes performance really bad.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>