mhi: core: Handle RSC minimum credit requirement

After getting OOB driver checks for transfer ring empty
condition and rings the channel ring doorbell without
making sure if there are minimum 8 TREs available for
GSI to perform DMA. Otherwise there is a possibility of
Host ringing doorbell after OOB with same write pointer
which was already processed in polling mode.

Change-Id: Ibf6dd14c12a2ab7955c5c70973c4ea9f5a38c0f2
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>