serial: sunsab: Don't enable tx if tx stopped

The serial core may call the UART driver's start_tx() even if
tx is stopped; the UART driver must verify tx should be enabled
before transmitting.

Reported-by: Sam Ravnborg <sam@ravnborg.org>
cc: David S. Miller <davem@davemloft.net>
cc: <sparclinux@vger.kernel.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>