platform/surface: aggregator: fix recv_buf() return value

platform/surface: aggregator: fix recv_buf() return value

Serdev recv_buf() callback is supposed to return the amount of bytes
consumed, therefore an int in between 0 and count.

Do not return negative number in case of issue, when
ssam_controller_receive_buf() returns ESHUTDOWN just returns 0, e.g. no
bytes consumed, this keep the exact same behavior as it was before.

This fixes a potential WARN in serdev-ttyport.c:ttyport_receive_buf().

Fixes: c167b9c7e3d6 ("platform/surface: Add Surface Aggregator subsystem")
Cc: stable@vger.kernel.org
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/20231128194935.11350-1-francesco@dolcini.it
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>