chromeos: Avoid controlling multiple mixers on x86.
chromeos: Avoid controlling multiple mixers on x86.

r94985 made the mixer code look for the Headphone and
Speaker controls present on ARM in addition to the Master
control that's not present there.  However, all three
controls are present on x86, and adjusting all three
simultaneously gives us lower volumes than desired.

This change makes us only use the Headphone and Speaker
controls if Master doesn't exist.

BUG=chromium-os:18578
TEST=manual: ran alsamixer and observed that volume keys now adjust only Master and PCM on x86-mario

Review URL: http://codereview.chromium.org/7550018

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@95247 0039d316-1c4b-4281-b951-d872f2087c98

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