ASoC: Tegra: Harmony: Explicitly set mic enables

Harmony has both an external mic (a regular mic jack) and an internal mic
(a 0.1" two-pin header on the board).

The external mic is connected to the WM8903's IN1L pin, and is supported
by the current driver.

The internal mic is connected to the WM8903's IN1R pin, and is not supported
by the current driver.

It appears that no Harmony systems were shipped with any internal mic
connected; users were expected to provide their own. This makes the
internal mic connection less interesting.

The WM8903's Mic Bias signal is used for both of these mics. For each mic,
a GPIO drives a transistor which gates whether the mic bias signal is
actively connected to that mic, or isolated from it.

The dual use of the mic bias for both mics makes a general-purpose complete
implementation of mic detection using the mic bias complex. So, for
simplicity, the internal mic is currently ignored by the driver.

This patch configures the relevant GPIOs to enable the mic bias connection
to the external mic, and disable the mic bias connection to the internal
mic. Note that in practice, this is the default state if these GPIOs aren't
configured.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>