mhi: dev: uci: block file open until MHI remove is complete

Race condition exists between open and remove where after open
obtain the device node and MHI remove frees the same device. To
avoid this scenario, block file open from obtaining device node
while remove is executing.

CRs-Fixed: 2356376
Change-Id: I1aad5c7848b791a2e03ea0acac643b9a2bda3ba2
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>