Bluetooth: btusb: remove set but not used variable in btusb_mtk_setup_firmware_79xx

Fix follow warning:
drivers/bluetooth/btusb.c:3479:9: warning: variable ‘fw_size’ set but not used [-Wunused-but-set-variable]
  size_t fw_size;
         ^~~~~~~
drivers/bluetooth/btusb.c:3473:29: warning: variable ‘patchhdr’ set but not used [-Wunused-but-set-variable]
  struct btmtk_patch_header *patchhdr = NULL;
                             ^~~~~~~~

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>