common/sfc_efx/base: support selecting RSS table entry count

common/sfc_efx/base: support selecting RSS table entry count

On Riverhead boards, the client can control how many entries
to have in the indirection table of an exclusive RSS context.

Provide the new parameter to clients and indicate its bounds.
Extend the API for writing the table to have the flexibility.

Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Reviewed-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>