net: wwan: common debugfs base dir for wwan device

This patch set brings in a common debugfs base directory
i.e. /sys/kernel/debugfs/wwan/ in WWAN Subsystem for a
WWAN device instance. So that it avoids driver polluting
debugfs root with unrelated directories & possible name
collusion.

Having a common debugfs base directory for WWAN drivers
eases user to match control devices with debugfs entries.

WWAN Subsystem creates dentry (/sys/kernel/debugfs/wwan)
on module load & removes dentry on module unload.

When driver registers a new wwan device, dentry (wwanX)
is created for WWAN device instance & on driver unregister
dentry is removed.

New API is introduced to return the wwan device instance
dentry so that driver can create debugfs entries under it.

Signed-off-by: M Chetan Kumar <m.chetan.kumar@linux.intel.com>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>