bus: mhi: core: Rename RDDM download function to use proper words

mhi_download_rddm_img() uses a shorter version of the word image.
Expand it and rename the function to mhi_download_rddm_image().

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>