bus: mhi: core: Add missing EXPORT_SYMBOL for mhi_get_mhi_state()

Add missing EXPORT_SYMBOL_GPL() declaration for mhi_get_mhi_state()
API.

Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>