soc: apple: rtkit: Crop syslog messages

Crop trailing whitespace, null, and newline characters in syslog
messages received from coprocessors. Notably DCP sends its messages
including a trailing newline, so prior to this change we would end up
cluttering the kernel log by repeated newlines at the end of messages.

Signed-off-by: Martin Povišer <povik+lin@cutebit.org>
Reviewed-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Hector Martin <marcan@marcan.st>