i2c_imx: Fix system fault when there is no data to send

When there is no data to send (e.g. i2c message with NULL buffer and
len=0), i2c_imx driver locks itself in isr handling forever.
So if there is no data to send, only device's address must be written
on bus. This fixes i2c_shell's scan command on both imx7 and imx6sx.

Signed-off-by: Antonio Tessarolo <anthonytexdev@gmail.com>