genesys: Fix [-Wmisleading-indentation] compiler warning

The fix is based on similar code in backend/genesys_gl846.c and fixes
a potential infinite loop as well.

backend/genesys_gl847.c

@@ -657,7 +657,7 @@ gl847_set_ad_fe (Genesys_Device * dev, uint8_t set)
 
   /* wait for FE to be ready */
   status = sanei_genesys_get_status (dev, &val8);
-  while (val8 & REG41_FEBUSY);
+  while (val8 & REG41_FEBUSY)
     {
       usleep (10000);
       status = sanei_genesys_get_status (dev, &val8);