linux_usbfs: Fix maybe-uninitialized error with -O3

Initialize active_config to an invalid value to avoid the following
compilation error:

os/linux_usbfs.c: In function ‘op_get_configuration’:
os/linux_usbfs.c:1452:12: error: ‘active_config’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
 1452 |  *config = (uint8_t)active_config;

Closes #1062
Closes #1063

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>