CRAS: alsa_io - Remove redundant overrun check.
CRAS: alsa_io - Remove redundant overrun check.

The overrun condition is checked by 'cras_shm_check_write_overrun'
before trying to fill the buffer, no need to double check here.

BUG=chromium-os:33123
TEST=unit tests still pass, aplay/arecord still work.

Change-Id: Ia07074218aa847716b569c014e781e4659edef58
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/29289

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