IB/hfi1: Limit VNIC use of SDMA engines to the available count

VNIC assumes that all SDMA engines have been configured for use.  This is
not necessarily true (i.e. if the count was constrained by the module
parameter).

Update VNICs usage to use the configured count, rather than the hardware
count.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Gary Leshner <gary.s.leshner@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: Jason Gunthorpe <jgg@mellanox.com>