if_bnxt: Convert all doorbell writes into function pointers

This is preparatory patch for making a base for Broadcom's Thor
controller support. It converts all doorbell writes into function
pointers.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D36437

(cherry picked from commit 49a3df78c75f29dfa4991314535c0eac4f5a73fa)