usbip: dump the port status difference

At the beginning of vhci_hub_control() function the port
status is saved to show what are the differences between the
previous and the new status. Change the dump function to
really show the differences in the status bit changes: put
a '+' sign before the symbolic name of a bit where a bit
was set, a '-' sign where the bit was cleared and a SPACE
where the bit was not changed. This is similar to the way
"diff -u" shows the changes and easy to understand.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>