IB/hfi1: Handle initial value of 0 for CCTI setting

When the driver is loaded it sets the default CCTI value to be 0. When the FM
starts and CCA is disabled the driver sets the max value to 65535 due the driver
subtracting 1 from 0 and the fact that the CCTI value is a u16.

Special case the subtraction to find the index for a 0 value.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>