msm: mhl: sii8334: Add mutex lock to avoid race condition

Add mutex lock betweeen mhl_sii_config and mhl_sii_device_discovery
functions to avoid accesing the variable flag before it is updated.
This will avoid accessing the mhl init settings i2c register
before turning on the mhl regulator's and gpio's.

Change-Id: If3f7fbe3d6ab84f101c407c43ac8b410e6df38fe
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>