sound: fix check for error condition of register_chrdev()

init_oss_soundcore() compares returned value of register_chrdev()
with -1, while other error codes can be returned.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Takashi Iwai <tiwai@suse.de>