bus: mhi: core: Remove unnecessary counter from mhi_firmware_copy()

There is an extra 'i' counter in the mhi_firmware_copy() function
which is unused. Remove it to clean-up code and reduce stack
space as well as improve efficiency of the function.

Fixes: cd457afb ("bus: mhi: core: Add support for downloading firmware over BHIe")
Reported-by: kernel test robot <lkp@intel.com>
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>