IB/qib: fix test of unsigned variable

Commit d4988623

 ("IB/qib: use arch_phys_wc_add()")
adjusted mtrr inititialization to use the new interface.

Unfortunately, the new interface returns a signed
value and the patch tested the unsigned wc_cookie.

Fix the issue by changing the type of wc_cookie to int.  For
the success case the ret left at zero to avoid
a warning from the caller.  For failure wc_cookie
is used as the ret.

Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>