cris: sync_serial: remove interruptible_sleep_on

sleep_on and its variants are racy and going away. This replaces
the two uses in the cris sync_serial drivers with the equivalent
but race-free wait_event_interruptible.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Mikael Starvik <starvik@axis.com>
Cc: linux-cris-kernel@axis.com
Signed-off-by: Jesper Nilsson <jespern@axis.com>