IB/hfi1: Don't modify num_user_contexts module parameter

The driver parameter num_user_contexts controls global behavior and
should not be modified by the driver.
This patch eliminates modification of num_user_contexts by using a
local variable to keep track of the value.

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