net: wan: framer: Add version sysfs attribute for the Lantiq PEF2256 framer

Lantiq PEF2256 framer has some little differences in behaviour
depending on its version.

Add a sysfs attribute to allow user applications to know the
version.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://patch.msgid.link/77a27941d6924b1009df0162ed9f0fa07ed6e431.1758726302.git.christophe.leroy@csgroup.eu


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