ALSA: es18xx: Add GPO controls

Add GPO0 and GPO1 (General Purpose Outputs) controls to mixer.
These can be used on some cards to control amplifier mute (seen in ES1868
datasheet) or additional onboard chips such as QX2130 QXpander processor.

These GPOs are present on ES1868, ES1869, ES1887 and ES1888 chips.

Tested on ES1868 with QX2130.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
ALSA: usb-audio: Flatten probe and disconnect functions

The usb-audio probe and disconnect functions have been split just for
adapting the (new!) API at 2.5 kernel time.  We left them until now,
partly because we wanted to build with the pretty old kernels in the
external alsa-driver tree.  But the support of such old kernels has
been longly stopped, so it's good time to clean up this mess.

One good point by this cleanup is that now the probe function returns
a proper error code instead of only -EIO.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
ALSA: usb-audio: Pass direct struct pointer instead of list_head

Some functions in mixer.c and endpoint.c receive list_head instead of
the object itself.  This is not obvious and rather error-prone.  Let's
pass the proper object directly instead.

The functions in midi.c still receive list_head and this can't be
changed since the object definition isn't exposed to the outside of
midi.c, so left as is.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
ALSA: usb-audio: Use strim() instead of open code

Signed-off-by: Takashi Iwai <tiwai@suse.de>