ksz884x: Remove unused functions

Remove unused functions.

These functions may have some value in documenting the
hardware. But that information may be accessed via SCM history.

Flagged by clang-16 with W=1.
No functional change intended.
Compile tested only.

Signed-off-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>