net: pse-pd: set user byte command SUB2 field

net: pse-pd: set user byte command SUB2 field

The Set User Byte to Save command has three subject bytes.
The PD692x0 protocol guides defines SUB2 with value 0x4e, while SUB1
carries the NVM user byte.

Template only initialized SUB and SUB1.
Fill SUB2 explicitly so the command matches the documented layout.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Acked-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://patch.msgid.link/20260611102517.445549-1-robert.marko@sartura.hr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>