alx: treat flow control correctly in alx_set_pauseparam()

Even when alx_setup_speed_duplex() is called, we still
need to call alx_cfg_mac_flowcontrol() and set hw->flowctrl
if flow control changed.

This was a bug I accidentally introduced while simplifying
the original driver.

Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>