net: usb: lan78xx: move functions to avoid forward definitions

Move following functions to avoid forward declarations in the code:
- lan78xx_start_hw()
- lan78xx_stop_hw()
- lan78xx_flush_fifo()
- lan78xx_start_tx_path()
- lan78xx_stop_tx_path()
- lan78xx_flush_tx_fifo()
- lan78xx_start_rx_path()
- lan78xx_stop_rx_path()
- lan78xx_flush_rx_fifo()

These functions will be used in an upcoming PHYlink migration patch.

No modifications to the functionality of the code are made.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20241204084142.1152696-4-o.rempel@pengutronix.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>