lascar-el-usb: lascar_scan(): Fix USB device list leak

lascar-el-usb: lascar_scan(): Fix USB device list leak

lascar_scan() calls libusb_get_device_list() but never the matching
libusb_free_device_list(). This will leak the memory allocated for the
device list as well as all the devices. To fix this add the missing
libusb_free_device_list().

While we are at it also make sure to handle errors returned by
libusb_get_device_list().

The issue was discovered using the following coccinelle semantic patch:
// <smpl>
@@
identifier devlist;
expression ctx, ret;
statement S;
@@
(
 libusb_get_device_list(ctx, &devlist);
|
 ret = libusb_get_device_list(ctx, &devlist);
 if (ret < 0) S
)
... when != libusb_free_device_list(devlist, ...)
*return ...;
// </smpl>

Signed-off-by: Lars-Peter Clausen <redacted>