net: ethtool: check rxfh_max_num_contexts != 1 at register time

A value of 1 doesn't make sense, as it implies the only allowed
 context ID is 0, which is reserved for the default context - in
 which case the driver should just not claim to support custom
 RSS contexts at all.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://lore.kernel.org/c07725b3a3d0b0a63b85e230f9c77af59d4d07f8.1723045898.git.ecree.xilinx@gmail.com


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