usb: wwan: fix compilation without CONFIG_PM_RUNTIME

The pm usage counter must be accessed with the proper wrappers
to allow compilation under all configurations.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Reported-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>