mhi: core: Use appropriate names for BHI/BHIe firmware load functions

mhi_fw_load_sbl() function is currently used to transfer SBL or EDL
images over BHI (Boot Host Interface). Same goes with mhi_fw_load_amss()
which uses BHIe. However, the contents do not indicate support for a
specific set of images. Since these can be used for any image download
over BHI or BHIe, we should rename them based on the protocol used.

Change-Id: I108aec270c0ae915cae5e08710402962a723cf0f
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>