eth: bnxt: allocate enough buffer space to meet HDS threshold

Now that we can configure HDS threshold separately from the rx_copybreak
HDS threshold may be higher than rx_copybreak.

We need to make sure that we have enough space for the headers.

Fixes: 6b43673a ("bnxt_en: add support for hds-thresh ethtool command")
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Link: https://patch.msgid.link/20250119020518.1962249-7-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>