eth: bnxt: fix backward compatibility with older devices

eth: bnxt: fix backward compatibility with older devices

Recent FW interface update bumped the size of struct hwrm_func_cfg_input
above 128B which is the max some devices support.

Probe on Stratus (BCM957452) with FW 20.8.3.11 fails with:

   bnxt_en ...: Unable to reserve tx rings
   bnxt_en ...: 2nd rings reservation failed.
   bnxt_en ...: Not enough rings available.

Once probe is fixed other errors pop up:

   bnxt_en ...: Failed to set async event completion ring.

This is because __hwrm_send() rejects requests larger than
bp->hwrm_max_ext_req_len with -E2BIG. Since the driver doesn't
actually access any of the new fields, yet, trim the length.
It should be safe.

Similar workaround exists for backing_store_cfg_input.
Although that one mins() to a constant of 256, not 128
we'll effectively use here. Michael explains: "the backing
store cfg command is supported by relatively newer firmware
that will accept 256 bytes at least."

To make debugging easier in the future add a warning
for oversized requests.

Fixes: 754fbf604ff6 ("bnxt_en: Update firmware interface to 1.10.2.171")
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/20231016171640.1481493-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>