bnxt_en: Improve RSS context reservation infrastructure

Add RSS context fields to struct bnxt_hw_rings and struct bnxt_hw_resc.
With these, we can now specific the exact number of RSS contexts to
reserve and store the reserved value.  The original code relies on
other resources to infer the number of RSS contexts to reserve and the
reserved value is not stored.  This improved infrastructure will make
the RSS context accounting more complete and is needed by later
patches.

Signed-off-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>