USB: serial: keyspan: fix debug and error messages

The URB status is signed and should be printed using %d rather than %x.

Also print endpoint addresses consistently using %x rather than %d, and
merge a broken-up error message string.

Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>