[ALSA] Don't reject O_RDWR at opening PCM OSS with read/write-only device

Accept O_RDWR at opening a PCM OSS device that is read- or write-only,
just for the compatibility with the behavior of older versions.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>