samples: ipc: icmsg: Do not return busy error codes

When sending a lot of data, a busy error code returned by the IPC is to
be expected. But if the last attempted send returns busy, this error
code is returned to the calling function, causing an error log message.
Fix this by setting return variable to 0 on this acceptable error.

Signed-off-by: Celina Sophie Kalus <hello@celinakalus.de>