bnxt_en: Refactor completion ring allocation logic for P5_PLUS chips

bnxt_en: Refactor completion ring allocation logic for P5_PLUS chips

Add a new bnxt_hwrm_cp_ring_alloc_p5() function to handle allocating
one completion ring on P5_PLUS chips.  This simplifies the existing code
and will be useful later in the series.

Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://patch.msgid.link/20250213011240.1640031-3-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>