slip: Fix deadlock in write_wakeup

      Use schedule_work() to avoid potentially taking the spinlock in
interrupt context.

Commit cc9fa74e2a ("slip/slcan: added locking in wakeup function") added
necessary locking to the wakeup function and 367525c8c2/ddcde142be ("can:
slcan: Fix spinlock variant") converted it to spin_lock_bh() because the lock
is also taken in timers.

Disabling softirqs is not sufficient, however, as tty drivers may call
write_wakeup from interrupt context. This driver calls tty->ops->write() with
its spinlock held, which may immediately cause an interrupt on the same CPU and
subsequent spin_bug().

Simply converting to spin_lock_irq/irqsave() prevents this deadlock, but
causes lockdep to point out a possible circular locking dependency
between these locks:

(&(&sl->lock)->rlock){-.....}, at: slip_write_wakeup
(&port_lock_key){-.....}, at: serial8250_handle_irq.part.13

The slip transmit is holding the slip spinlock when calling the tty write.
This grabs the port lock.