network-manager: fix getting of ifname from the sysfs path

network-manager: fix getting of ifname from the sysfs path

commit 5e0f8c8a4ced ('network-manager: remove useless use of basename')
somewhat carelessly didn't take into account that $_i has a slash at
the end which made the result of the ## substitution be just an empty
string.

The slash was put to the end of /sys/class/net/*/ to make sure we're only
iterating directories, but it's not strictly necessary. In an unlikely case
something else than a directory appears in /sys/class/net/, we'll already deal
with it gracefully. Remove it.

This fixes the TEST-30-ISCSI test.