bnxt_en: add debugfs support for DIM

This adds debugfs support for bnxt_en with the purpose of allowing users
to examine the current DIM profile in use for each receive queue.  This
was instrumental in debugging issues found with DIM and ensuring that
the profiles we expect to use are the profiles being used.

Signed-off-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>