rambi: fix two bugs with accelerometer driver
rambi: fix two bugs with accelerometer driver

Two bugs are:
- Need to grab the I2C port mutex before performing i2cxfer.
- Added sending software reset command to accelerometers on init.
This is necessary because the accelerometers can be powered
through an EC reboot, and it's important we restore them to a
known state.

BUG=none
BRANCH=rambi
TEST=Manually set accelerometer setting, rebooted, manually read
that setting and verified it was restored to default.

Change-Id: Ic3034ae39c936e07ca28458a60557b9623674ff1
Original-Change-Id: I0ea571f3a8dc46052128def24cbb5c1c29638469
Signed-off-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/188349
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/188387

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