mhi: dev: net_dev: remove pm_lock since it's not needed

napi_disable() flushes any napi threads that are scheduled before
returning. Therefore, we don't need a pm_lock in the driver to
protect the driver states.

CRs-Fixed: 2361266
Change-Id: I1f03f14dd0f58f1b6ff1c4e2d50af1900427e307
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>