nvme-topology: fix returning invalid value in scan_subsystems()

nvme-topology: fix returning invalid value in scan_subsystems()

Let's say we have multiple three subsystems in the system.  If we run
a simple 'nvme list' command, then scan_subsystems() will never set the
'ret' value which is a local variable.  This should be initialized to a
initial value to avoid invalid value returning.

This patch fixes returning invalid value from the scan_subsystems() in
case of multi-subsystems found.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>