Bluetooth: hci_intel: Fix firmware file name to use hw_variant

The format of Intel Bluetooth firmware for bootloader product is
ibt-<hw_variant>-<device_revision_id>.sfi and .ddc.

This patch uses a hw_variant value read from the device during
runtime to form the firmware filenames instead of using a constant
value, so it can support multiple prouducts.

Signed-off-by: Tedd Ho-Jeong An <tedd.an@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>