serial: fix off by one errors

In zs_console_putchar() occurs:

	if (zs_transmit_drain(zport, irq))
		write_zsdata(zport, ch);

However if in zs_transmit_drain() no empty Tx Buffer occurs, limit reaches
-1 => true, and the write still occurs.

This patch changes postfix to prefix decrements in this and similar
functions to prevent similar mistakes in the future.  This decreases the
iterations with one but the chosen loop count was arbitrary anyway.

In sunhv limit reaches -1, not 0, so the test is off by one.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>