USB: usb-skeleton.c: cleanup open_count

It is not necessary to use the 'open_count' for handling
runtime pm only, because runtinme pm has built-in counter
to handle this, so remove it to make code clean.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>