drivers/flash: stm32g0: Prepare for unaligned accesses in flash writes

When using the settings subsystem, the data argument argument passed to
flash_stm32_write_range() might not be 8-bytes aligned, causing an
unaligned memory access fault.

Fix that the same way as it was done for the STM32L4 in commit
652efa53.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>