net: pcs: xpcs: remove switch() in xpcs_link_up_1000basex()

Remove an unnecessary switch() statement in xpcs_link_up_1000basex().
The only value this switch statement is interested in is SPEED_1000,
all other values lead to an error. Replace this with a simple if()
statement.

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>