net: pcs: xpcs: Use devm_clk_get_optional

Synopsys DesignWare XPCS CSR clock is optional,
so it is better to use devm_clk_get_optional
instead of devm_clk_get.

Signed-off-by: Jack Ping CHNG <jchng@maxlinear.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250715021956.3335631-1-jchng@maxlinear.com


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