bnxt_en: Refactor the functions to reserve hardware rings.

The bnxt_hwrm_reserve_{pf|vf}_rings() functions are very similar to
the bnxt_hwrm_check_{pf|vf}_rings() functions.  Refactor the former
so that the latter can make use of common code in the next patch.

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