usb: dwc3-msm: Clean up gsi_prepare_trbs()

This is mostly a cosmetic change to clean up gsi_prepare_trbs()
for better readability:

 - Perform a memset() once for the entire pool instead of
   zeroing out each TRB within the loop.
 - Separate out the initialization for link TRBs and keep
   the for loops for just the normal buffer initialization

Finally, remove the final loop at the end whose only purpose
is to print the entire TRB pool contents to the kernel log.
It's just as useful to simply inspect using debugfs as every
endpoint already offers a trb_ring file.

Change-Id: Ifcd841efbd51cc3229a3d4e552a3df1ef244ce96
Signed-off-by: Jack Pham <jackp@codeaurora.org>