mhi: dev: uci: exit immediately if MHI channel is in disabled state

If write thread woke up because device is in a disabled state,
exit immediately and return error to user.

CRs-Fixed: 2258304
Change-Id: I7de224e29d0ef9b414a014ef0d874156367d0cb6
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>