mlxsw: Fix the size of 'VIRT_ROUTER_MSB'

The field 'virtual router' was extended to 12 bits in Spectrum-4.
Therefore, the element 'MLXSW_AFK_ELEMENT_VIRT_ROUTER_MSB' needs 3 bits for
Spectrum < 4 and 4 bits for Spectrum >= 4.

The elements are stored in an internal storage scratchpad. Currently, the
MSB is defined there as 3 bits. It means that for Spectrum-4, only 2K VRFs
can be used for multicast routing, as the highest bit is not really used by
the driver. Fix the definition of 'VIRT_ROUTER_MSB' to use 4 bits. Adjust
the definitions of 'virtual router' field in the blocks accordingly - use
'_avoid_size_check' for Spectrum-2 instead of for Spectrum-4. Fix the mask
in parse function to use 4 bits.

Fixes: 6d5d8ebb

 ("mlxsw: Rename virtual router flex key element")
Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/79bed2b70f6b9ed58d4df02e9798a23da648015b.1692268427.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>