bnxt_en: add support for rx-copybreak ethtool command

bnxt_en: add support for rx-copybreak ethtool command

The bnxt_en driver supports rx-copybreak, but it couldn't be set by
userspace. Only the default value(256) has worked.
This patch makes the bnxt_en driver support following command.
`ethtool --set-tunable <devname> rx-copybreak <value> ` and
`ethtool --get-tunable <devname> rx-copybreak`.

By this patch, hds_threshol is set to the rx-copybreak value.
But it will be set by `ethtool -G eth0 hds-thresh N`
in the next patch.

Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Brett Creeley <brett.creeley@amd.com>
Tested-by: Stanislav Fomichev <sdf@fomichev.me>
Tested-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Link: https://patch.msgid.link/20250114142852.3364986-7-ap420073@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>