diag: Increase number of buffers for MHI and USB

When there is heavy diag traffic, all the read buffers
for MHI are used. This change doubles the amount of buffers to
increase the packet throughput.

CRs-Fixed: 971043
Change-Id: I854abe0ba5273add8dee6a9762598706db12804a
Signed-off-by: Chris Lew <clew@codeaurora.org>