input: sensors: fix MPU6050 mutex deadlock

MPU6050 driver uses a single global mutex to prevent reentrance
but this mutex can cause deadlock when flush workqueues. This
change removes mutex lock from workqueue function to fix this
issue.

Change-Id: Ifa194401d2e7a6a5cacd2d2337b556f000d35974
Signed-off-by: Bingzhe Cai <bingzhec@codeaurora.org>