drivers: i2c: sam: fix the exception when transferring without data

The issue is found when doing shell command "i2c scan" on sama7g54-ek.
In this case no data will be transferred besides START and STOP. Data
abort would occur on accessing "msg->buf[msg->idx++]" when MMU is
enabled and "msg->idx" is very large.

Signed-off-by: Tony Han <tony.han@microchip.com>