staging: octeon-ethernet: eliminate USE_10MBPS_PREAMBLE_WORKAROUND define

We have the workaround always enabled, so eliminate a redundant #define.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>