net: fec: fix build error in fec driver

commit f4d8124a which is commit 
a069215c upstream caused a build error
in the driver, as the pm functions were not included properly.  So fix
that by including the needed .h file.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>