drivers: auxdisplay: Fix text direction handling

Fixed Input Set handling.
Flag had misleading names: whether "entry" happens to the left or to the
right is dependent on whether increment mode is active or not), so
switched to just using a "SHIFT" y/n flag instead. It also reflects better
the contents of the datasheet).
Updated romance text direction accordingly ("increment, no shift").

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>