dfu: check interface class when opening DFU interfaces

dfu_rtd_open() and dfu_moded_open() matched interface only by subclass
and protocol. Bluetooth primary controller interface uses the same
values (class 0xE0, subclass 0x01, protocol 0x01) as DFU runtime, so
in a device with both BTH and DFU runtime the DFU runtime driver,
which comes first in driver list, claimed the Bluetooth interface and
SET_CONFIGURATION failed.

Check that interface class is application specific (0xFE) as well.

Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Jerzy Kasenberg <jerzy@apache.org>