mei: don't update offset in write

commit a103af1b64d74853a5e08ca6c86aeb0e5c6ca4f1 upstream.

MEI enables writes of complete messages only
while read can be performed in parts, hence
write should not update the file offset to
not break interleaving partial reads with writes.

Cc: <stable@vger.kernel.org>
Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>