sfc: avoid division by zero

The division is already being done properly in efx_ef10_get_timer_config
which returns zero-on-success, unlike the old efx_ef10_get_sysclk_freq.

Fixes: d95e329a

 ("sfc: get timer configuration from adapter")
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>