usb: gadget: f_gsi: Use 16KB buffer with each TRB for MBIM DL

There is limitation when IPA is performing de/aggregation with MBIM
in scatter-gather mode. Hence instead of 2KB buffer use 16KB buffer
(i.e. size of aggregated frame) with each TRB for MBIM DL case.

CRs-Fixed: 1032467
Change-Id: I4f73d30711b5606c2ac88ce9d05a8483d64b665d
Signed-off-by: Mayank Rana <mrana@codeaurora.org>