net: phylink: use for_each_set_bit()

Use for_each_set_bit() rather than open coding the for() test_bit()
loop.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Link: https://lore.kernel.org/r/E1r4p15-00Cpxe-C7@rmk-PC.armlinux.org.uk


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