staging: et131x: Remove yet more forward declarations

Moved functions in et131x.c file to remove the forward declarations of:

et1310_setup_device_for_multicast
et1310_setup_device_for_unicast
et131x_up
et131x_down
et131x_enable_txrx
et131x_disable_txrx

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>