mei: demote client disconnect warning on suspend to debug

Change level for the "not connected" client message in the write
callback from error to debug.

The MEI driver currently disconnects all clients upon system suspend.
This behavior is by design and user-space applications with
open connections before the suspend are expected to handle errors upon
resume, by reopening their handles, reconnecting,
and retrying their operations.

However, the current driver implementation logs an error message every
time a write operation is attempted on a disconnected client.
Since this is a normal and expected flow after system resume
logging this as an error can be misleading.

Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Link: https://lore.kernel.org/r/20240530091415.725247-1-tomas.winkler@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>