imx_serial: Generate interrupt on tx empty if enabled

imx_serial: Generate interrupt on tx empty if enabled

Generate an interrupt if the tx buffer is empty and the tx empty interrupt
is enabled. This fixes a problem seen when running a Linux image since
Linux commit 55c3cb1358e ("serial: imx: remove unneeded imx_transmit_buffer()
from imx_start_tx()"). Linux now waits for the tx empty interrupt before
starting to send data, causing transmit stalls until there is an interrupt
for another reason.

Signed-off-by: Guenter Roeck <[email protected]>
Reviewed-by: Peter Crosthwaite <[email protected]>
Signed-off-by: Michael Tokarev <[email protected]>