drivers: ethernet: stm32: Disable HW checksums by default

Without this change, there's a difference between what the V2 Ethernet
HAL claims to be its capabilities in eth_stm32_hal_get_capabilities()
and what is actually enabled when CONFIG_ETH_STM32_HW_CHECKSUM is
disabled. This difference somehow causes the checksum in outgoing
packets to become 0, and hence seems to break networking at least on
some PHYs.

This commit disables HW checksums by default even when V2 driver is in
use, and this hence fixes networking when CONFIG_ETH_STM32_HW_CHECKSUM
is disabled.

This fixes #57629.

Signed-off-by: Ole Morten Haaland <omh@icsys.no>