ALSA: opl3: Kill unused set_fs()

snd_enter_user() and snd_leave_user() that call set_fs() are the dead
code in opl3 driver.  Let's rip them off.

Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>