ASoC: pcm3060: Rename output widgets

In the initial commit [1], I added differential output of the codec as
separate `+` and `-` widgets:

OUTL+
OUTR+
OUTL-
OUTR-

Later, in the commit [2], I added a device tree property to configure the
output as single-ended or differential. Having this property, the `+` and
`-` separation in widgets seems for me confusing. There are no functional
benefits in such separation, so I find reasonable to get rid of it:

OUTL
OUTR

The new naming is more friendly for sound cards, and is better aligned with
other codec drivers in kernel.

Renaming the output widgets now should not be a problem from the backwards-
compatibility perspective, as the driver for PCM3060 is added into the
mainline very recently, and did not yet appear in any releases.

[1] commit 6ee47d4a

 ("ASoC: pcm3060: Add codec driver")
[2] commit a78c62de00d5 ("ASoC: pcm3060: Add DT property for single-ended
    output")

Signed-off-by: Kirill Marinushkin <kmarinushkin@birdec.tech>
Signed-off-by: Mark Brown <broonie@kernel.org>