net: sfc: remove redundant variable start

Variable start is assigned but never read hence it is redundant
and can be removed. Cleans up clang warning:

drivers/net/ethernet/sfc/ptp.c:655:2: warning: Value stored to 'start'
is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>