net: pcs: xpcs: remove return statements in void function

While using "return" when calling a void returning function inside a
function that returns void doesn't cause a compiler warning, it looks
weird. Convert the bunch of if() statements to a switch() and remove
these return statements.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>