mlxsw: Verify the accessed index doesn't exceed the array length

There are few cases in which an array index queried from a fw register,
is accessed without any validation that it doesn't exceed the array
length.

Add a proper length validation, so accessing memory past the end of an
array will be forbidden.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>