staging: et131x: Remove more forward declarations

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

et131x_soft_reset
et131x_isr_handler
et131x_device_alloc
Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>