net: net_failover: Correct parameter name 'standby_dev'

net: net_failover: Correct parameter name 'standby_dev'

Fixes the following W=1 kernel build warning(s):

 drivers/net/net_failover.c:711: warning: Function parameter or member 'standby_dev' not described in 'net_failover_create'
 drivers/net/net_failover.c:711: warning: Excess function parameter 'dev' description in 'net_failover_create'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201102114512.1062724-30-lee.jones@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>