mei: bus: add mei_cldev_mtu interface

mei: bus: add mei_cldev_mtu interface

Add a new helper function that allows MEI client drivers
to query the maximum transmission unit (MTU) for a connected
MEI client.

This is useful for clients that need to transmit large payloads,
such as firmware blobs, allowing them to determine the maximum
message size that can be safely sent before starting transmission and
size of the buffer to allocate when receiving data.

Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Badal Nilawar <badal.nilawar@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250905154953.3974335-2-badal.nilawar@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>