cfg802154: pass name_assign_type to rdev_add_virtual_intf()

This code is based on commit 6bab2e19
("cfg80211: pass name_assign_type to rdev_add_virtual_intf()")

This will expose in sysfs whether the ifname of a IEEE-802.15.4
device is set by userspace or generated by the kernel.
We are using two types of name_assign_types
 o NET_NAME_ENUM: Default interface name provided by kernel
 o NET_NAME_USER: Interface name provided by user.

Signed-off-by: Varka Bhadram <varkab@cdac.in>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>