selftests: mlxsw: qos_mc_aware: Specify arping timeout as an integer

Starting from iputils s20190709 (used in Fedora 31), arping does not
support timeout being specified as a decimal:

$ arping -c 1 -I swp1 -b 192.0.2.66 -q -w 0.1
arping: invalid argument: '0.1'

Previously, such timeouts were rounded to an integer.

Fix this by specifying the timeout as an integer.

Fixes: a5ee171d

 ("selftests: mlxsw: qos_mc_aware: Add a test for UC awareness")
Signed-off-by: Amit Cohen <amitc@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>