ptp: ocp: Document sysfs output format for backward compatibility

ptp: ocp: Document sysfs output format for backward compatibility

Add a comment to ptp_ocp_tty_show() explaining that the sysfs output
intentionally does not include a trailing newline. This is required for
backward compatibility with existing userspace software that reads the
sysfs attribute and uses the value directly as a device path.

A previous attempt to add a newline to align with common kernel
conventions broke userspace applications that were opening device paths
like "/dev/ttyS4\n" instead of "/dev/ttyS4", resulting in ENOENT errors.

This comment prevents future attempts to "fix" this behavior, which would
break existing userspace applications.

Link: https://lore.kernel.org/netdev/20251030124519.1828058-1-zhongqiu.han@oss.qualcomm.com/
Link: https://lore.kernel.org/netdev/aef3b850-5f38-4c28-a018-3b0006dc2f08@linux.dev/
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20251107074533.416048-1-zhongqiu.han@oss.qualcomm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>