drivers: ethernet: Update ETH_LAN865X_TIMEOUT Kconfig description

The description is a bit misleading as the packet is not even read in
the mentioned case by the OA TC6 Zephyr driver.

When the timeout occurs the data (packet) received by LAN865x may be:
- Read latter if still in the RX buffer of LAN865x
or
- Is (probably) dropped by LAN8651 itself as the RX buffer gets overrun

Signed-off-by: Lukasz Majewski <lukma@denx.de>