sisusbvga: Fix oops on disconnect.

Remove dev_info call on disconnect. The sisusb_dev pointer may have been
set to zero by sisusb_delete at this point causing an oops.

The message does not provide any extra information over the standard USB
subsystem output so removing it does not affect functionality.

Signed-off-by: Will Newton <will.newton@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>