net: atlantic: proper rss_ctrl1 (54c0) initialization

This patch fixes an inconsistency between code and spec, which
was found while working on the QoS implementation.

When 8TCs are used, 2 is the maximum supported number of index bits.
In a 4TC mode, we do support 3, but we shouldn't really use the bytes,
which are intended for the 8TC mode.

Signed-off-by: Mark Starovoytov <mstarovoitov@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>