selftests: drv-net: xdp: make sure we're actually testing native XDP

selftests: drv-net: xdp: make sure we're actually testing native XDP

Kernel tries to be helpful and attach the XDP program in generic
mode if the driver has no BPF ndo at all. Since the xdp.py tests
all have "native" in their names this can be quite confusing.
Force native / "drv" attachment. Note that netdevsim re-uses
the generic handler as its "native" handler, so we'll maintain
the test coverage of the generic mode that way. No need to test
both explicitly, I reckon.

Link: https://patch.msgid.link/20250822195645.1673390-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>