linux_usbfs: Accept sysfs attributes not terminated with newline

linux_usbfs: Accept sysfs attributes not terminated with newline

Benjamin Berg reports that some CI systems that simulate sysfs devices
are causing libusb to report errors because such systems are not
conforming to the sysfs pattern of terminating attribute values with a
newline character. Reduce the severity of encountering such
non-conforming attibute values from an error that prevents enumeration
to a warning message.

Closes #857

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>