spi: loopback-test: fix up const pointer issue in rx_ranges_cmp()

When a list head is a const pointer, the list entry for that head also
must remain a const pointer, otherwise we are just "throwing it away"
for no good reason.  Fix this up by properly marking these structures as
const.

Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patch.msgid.link/2025052213-semifinal-sublevel-d631@gregkh


Signed-off-by: Mark Brown <broonie@kernel.org>