Bluetooth: btusb: QCA: Fix downloading wrong NVM for WCN6855 GF variant without board ID

For GF variant of WCN6855 without board ID programmed
btusb_generate_qca_nvm_name() will chose wrong NVM
'qca/nvm_usb_00130201.bin' to download.

Fix by choosing right NVM 'qca/nvm_usb_00130201_gf.bin'.
Also simplify NVM choice logic of btusb_generate_qca_nvm_name().

Fixes: d6cba4e6 ("Bluetooth: btusb: Add support using different nvm for variant WCN6855 controller")
Signed-off-by: Zijun Hu <zijun.hu@oss.qualcomm.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>