serial: pmac_zilog: push received data to the port that received it

pmz_interrupt() drains both channels but, since commit 2e124b4a390c
("TTY: switch tty_flip_buffer_push"), pushes the flip buffer of the
port whose handler is running rather than the port that was drained.
With both ports open, data received on the other port can be left in
its flip buffer until something else pushes it, so its reader may not
see it.

Fixes: 2e124b4a390c ("TTY: switch tty_flip_buffer_push")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Tim Douglas <me@timdoug.com>
Link: https://patch.msgid.link/20261013013541.4535-2-me@timdoug.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>