eth: mlx5: migrate to the *_rxfh_context ops

eth: mlx5: migrate to the *_rxfh_context ops

Convert mlx5 to dedicated RXFH ops. This is a fairly shallow
conversion, TBH, most of the driver code stays as is, but we
let the core allocate the context ID for the driver.

mlx5e_rx_res_rss_get_rxfh() and friends are made void, since
core only calls the driver for context 0. The second call
is right after context creation so it must exist (tm).

Tested with drivers/net/hw/rss_ctx.py on MCX6.

Reviewed-by: Gal Pressman <gal@nvidia.com>
Link: https://patch.msgid.link/20250707184115.2285277-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>