mlxsw: Rename virtual router flex key element

In Spectrum-4, the size of the virtual router ACL key element increased
from 11 bits to 12 bits.

In order to reuse the existing virtual router ACL key element
enumerators for Spectrum-4, rename 'VIRT_ROUTER_8_10' and
'VIRT_ROUTER_0_7' to 'VIRT_ROUTER_MSB' and 'VIRT_ROUTER_LSB',
respectively.

No functional changes intended.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>