net: phy: fixed: allow setting no update_link callback

fixed_phy_set_link_update() contains an early check against a NULL
callback pointer, which basically prevents us from removing any
previous callback we may have set. The users of the fp->link_update
callback deal with a NULL callback just fine, so we really want to allow
"removing" a link_update callback to avoid dangling callback pointers
during e.g: module removal.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>