drivers: net: ethernet: octeon_mgmt: fix a compiler warning

Fix the following compiler warning:

drivers/net/ethernet/octeon/octeon_mgmt.c: In function 'octeon_mgmt_clean_tx_buffers':
drivers/net/ethernet/octeon/octeon_mgmt.c:295:4: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
    u64 ns = cvmx_read_csr(CVMX_MIXX_TSTAMP(p->port));
    ^

Signed-off-by: Aaro Koskinen <aaro.koskinen@nsn.com>
Acked-by: David Daney <david.daney@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>