am7xxx: fix a clang warning

  ../src/am7xxx.c:685:16: error: implicit conversion loses integer precision: 'ssize_t' (aka 'long') to 'int'
        [-Werror,-Wshorten-64-to-32]
          num_devices = libusb_get_device_list(ctx->usb_context, &list);
                    ~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

libusb_get_device_list() returns ssize_t, just use it for the num_devices
variable.