drivers: modem: fix extra bytes sent by mdm_receiver_send()

mdm_receiver_send() was sending 1 too many bytes of buf.  This ended
up being the NULL terminator.  Size should be reduced prior to the
while check so that this doesn't happen.

Fixes: #14001

Signed-off-by: Michael Scott <[email protected]>

Author: mike-scott

drivers/modem/modem_receiver.c

@@ -188,7 +188,7 @@ int mdm_receiver_send(struct mdm_receiver_context *ctx,
 
 	do {
 		uart_poll_out(ctx->uart_dev, *buf++);
-	} while (size--);
+	} while (--size);
 
 	return 0;
 }