libztex: in case the selectFpga() failed set the selected fpga to unknown

after libztex_selectFpga() failed we can not be sure which fpga is now
selected. can be the old one or maybe even the new one. so set the
selected fpga variable to unknown so that the next call must set it
again.

libztex.c

@@ -535,6 +535,7 @@ int libztex_selectFpga(struct libztex_device *ztex)
 		cnt = libusb_control_transfer(ztex->root->hndl, 0x40, 0x51, (uint16_t)number, 0, NULL, 0, 500);
 		if (unlikely(cnt < 0)) {
 			applog(LOG_ERR, "Ztex check device: Failed to set fpga with err %d", cnt);
+			ztex->root->selectedFpga = -1;
 			return cnt;
 		}
 		ztex->root->selectedFpga = number;