mhi: dev: net_dev: remove support for dynamically changing transfer size

Maximum Receive Unit (MRU) is maximum packet size in bytes we can
receive in downlink direction. Removing support to set this size
dynamically since driver will always allocate maximum size
hardware support.

CRs-Fixed: 2361266
Change-Id: I78f34215b9e94cbbf3d5872eb31790108dbc14c0
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>