ltq-ptm: Fix netdev ioctls with kernel > 5.15
.ndo_do_ioctl is not called for SIOCDEVPRIVATE any more, the kernel
calls .ndo_siocdevprivate now.
The function gets the data pointer from the callback directly, make use
of it.
See upstream Linux kernel commit:
https://git.kernel.org/linus/
b9067f5dc4a07c8e24e01a1b277c6722d91be39e
Link: https://github.com/openwrt/openwrt/pull/16005
(cherry picked from commit
e33ebdd00e5aedacb996fd48987067c77e43f5f4)
Link: https://github.com/openwrt/openwrt/pull/16023
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>