ALSA: firewire-motu: drop EPOLLOUT from poll return values as write is not supported

ALSA: firewire-motu: drop EPOLLOUT from poll return values as write is not supported

The ALSA HwDep character device of the firewire-motu driver incorrectly
returns EPOLLOUT in poll(2), even though the driver implements no operation
for write(2). This misleads userspace applications to believe write() is
allowed, potentially resulting in unnecessarily wakeups.

This issue dates back to the driver's initial code added by a commit
71c3797779d3 ("ALSA: firewire-motu: add hwdep interface"), and persisted
when POLLOUT was updated to EPOLLOUT by a commit a9a08845e9ac ('vfs: do
bulk POLL* -> EPOLL* replacement("").').

This commit fixes the bug.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://patch.msgid.link/20250829233749.366222-1-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>