net: phy: fix sparse warning in fixed.c

This commit fixes the following sparse warning:

drivers/net/phy/fixed.c:207
    - warning: symbol 'fixed_phy_del' was not declared.
      Should it be static?

by adding symbol definition to the phy_fixed.h API file. It is ok to do
because the function in question is an exported symbol.

Signed-off-by: Konrad Zapalowicz <bergo.torino@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>