mhi: dev: netdev: use a single pool of buffer for all devices

On average, if MHI device consists of both rsc and legacy pipe,
only a single pipe is actively pumping data while other pipe is
in idle. To better utilize memory, use a single memory pool so
active channel can use a large memory pool.

CRs-Fixed: 2468499
Change-Id: I016e1625b911e8d6f6fefe389c2090a78f97aefa
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>