sfc: reduce the number of requested xdp ev queues

Without this change the driver tries to allocate too many queues,
breaching the number of available msi-x interrupts on machines
with many logical cpus and default adapter settings:

Insufficient resources for 12 XDP event queues (24 other channels, max 32)

Which in turn triggers EINVAL on XDP processing:

sfc 0000:86:00.0 ext0: XDP TX failed (-22)

Signed-off-by: Ivan Babrou <ivan@cloudflare.com>
Acked-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://lore.kernel.org/r/20210120212759.81548-1-ivan@cloudflare.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>