ALSA: opl3: fix mismatch between snd_opl3_drum_switch definition and declaration

The function snd_opl3_drum_switch declaration in the header file
has the order of the two arguments on_off and vel swapped when
compared to the definition arguments of vel and on_off.  Fix this
by swapping them around to match the definition.

This error predates the git history, so no idea when this error
was introduced.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>