sound: oss: midibuf: fix sleep_on races

sleep_on is known to be racy and going away because of this. All instances
of interruptible_sleep_on and interruptible_sleep_on_timeout in the midibuf
driver can trivially be replaced with wait_event_interruptible and
wait_event_interruptible_timeout.

[fixed coding style warnings by tiwai]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>