IB/hfi1: Size rcd array index correctly and consistently

The array index for the rcd array is sized several different ways
throughout the code.

Use the user interface size (u16) as the standard size and update the
necessary code to reflect this.

u16 is large enough for the largest amount of supported contexts.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>