USB: xhci - fix unsafe macro definitions

Macro arguments used in expressions need to be enclosed in parenthesis
to avoid unpleasant surprises.

This should be queued for kernels back to 2.6.31

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