slimbus: ngd: Optimize bulk write transactions

Allocate bulk-write transactions in advance for most-commonly
used bulk sizes, and reallocate only if size greater than that is
requested by client.

Change-Id: Ib5a80f3713fe5b94bd6dcf45ffe61e498fada192
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>