bus: mhi: core: Remove unused mhi_fw_load_worker() declaration

The mhi_fw_load_worker() function no longer exists. Remove its
declaration as part of code clean-up.

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>