gianfar: remove forward declarations

Remove forward declarations of various static functions located in two
driver implementation files and rearrange the corresponding definitions
accordingly.

This patch only introduces mechanical changes, namely it removes forward
declarations and moves function definitions around; it does not change any
functionality.

Signed-off-by: Arseny Solokha <asolokha@kb.kras.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>