ethtool: cmis_cdb: Fix incorrect read / write length extension

The 'read_write_len_ext' field in 'struct ethtool_cmis_cdb_cmd_args'
stores the maximum number of bytes that can be read from or written to
the Local Payload (LPL) page in a single multi-byte access.

Cited commit started overwriting this field with the maximum number of
bytes that can be read from or written to the Extended Payload (LPL)
pages in a single multi-byte access. Transceiver modules that support
auto paging can advertise a number larger than 255 which is problematic
as 'read_write_len_ext' is a 'u8', resulting in the number getting
truncated and firmware flashing failing [1].

Fix by ignoring the maximum EPL access size as the kernel does not
currently support auto paging (even if the transceiver module does) and
will not try to read / write more than 128 bytes at once.

[1]
Transceiver module firmware flashing started for device enp177s0np0
Transceiver module firmware flashing in progress for device enp177s0np0
Progress: 0%
Transceiver module firmware flashing encountered an error for device enp177s0np0
Status message: Write FW block EPL command failed, LPL length is longer
	than CDB read write length extension allows.

Fixes: 9a3b0d07 ("net: ethtool: Add support for writing firmware blocks using EPL payload")
Reported-by: Damodharam Ammepalli <damodharam.ammepalli@broadcom.com>
Closes: https://lore.kernel.org/netdev/20250402183123.321036-3-michael.chan@broadcom.com/


Tested-by: Damodharam Ammepalli <damodharam.ammepalli@broadcom.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Damodharam Ammepalli <damodharam.ammepalli@broadcom.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20250409112440.365672-1-idosch@nvidia.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>