mhi: core: change to vmalloc for memory allocations larger than 32KB

Certain large memory allocations are likely to exceed 32KB or
order of 3. It is recommended to switch to using vmalloc instead
to avoid possible failures while allocating memory.

CRs-Fixed: 2446873
Change-Id: I62ddc505956e116f55e8b3b974de3d5b039b0404
Acked-by: Bhaumik Vasav Bhatt <bbhatt@qti.qualcomm.com>
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>