ASoC: sun4i-codec: use consistent names for PA controls

The power amplifier for the headphone output is called "the PA" and "the
headphone amplifier" in Allwinner's documentation for the A10 and A20.
sun4i-codec calls it "PA" in some places and "Pre-Amplifier" (which
isn't really accurate) in others, leading to user-visible controls with
different names referring to the same device.

When this driver implements audio input, it'll also need to expose
controls for the line and mic input preamps, so just referring to "the
Pre-Amplifier" will be ambiguous.

Change it to use "Power Amplifier" consistently for the power
amplifier's controls.

Signed-off-by: Adam Sampson <ats@offog.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>