tty: mxser: drop low-latency workaround

Commit 67d2bc58afdd ("Char: mxser_new, fix recursive locking") worked
around the infamous low_latency behaviour of tty_flip_buffer_push() by
simply dropping and reacquiring the port lock in the interrupt handler.

Since commit a9c3f68f3cd8 ("tty: Fix low_latency BUG"),
tty_flip_buffer_push() always schedules a work item to push data to the
line discipline and there's no need to keep any low_latency hacks around.

Link: https://lore.kernel.org/lkml/[e-mail]/T/#m06b04c640a7b6f41afb3d34a4cf29b1df4935d3a
Reviewed-by: Jiri Slaby <[e-mail]>
Signed-off-by: Johan Hovold <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/mxser.c

···
 	port->mon_data.rxcnt += cnt;
 	port->mon_data.up_rxcnt += cnt;
 
-	/*
-	 * We are called from an interrupt context with &port->slock
-	 * being held. Drop it temporarily in order to prevent
-	 * recursive locking.
-	 */
-	spin_unlock(&port->slock);
 	tty_flip_buffer_push(&port->port);
-	spin_lock(&port->slock);
 }
 
 static void mxser_transmit_chars(struct tty_struct *tty, struct mxser_port *port)