diag: Avoid reallocating if buffer supports hdlc max pkt size

Instead of reallocating memory every time max pkt size for
hdlc encoding exceeds peripheral buffer size of 16K, reallocate
32k buffer once and reuse it.

Change-Id: I89ee577f2159dd68005447d12d8495f028b07d97
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>