[media] b2c2: use IS_REACHABLE() instead of open-coding it

The FE_SUPPORTED() macro is basically the same as IS_REACHABLE, except
that it causes a warning with gcc-7:

common/b2c2/flexcop-fe-tuner.c:30:1: error: this use of "defined" may not be portable [-Werror=expansion-to-defined]
common/b2c2/flexcop-fe-tuner.c:30:1: error: this use of "defined" may not be portable [-Werror=expansion-to-defined]
common/b2c2/flexcop-fe-tuner.c:30:1: error: this use of "defined" may not be portable [-Werror=expansion-to-defined]

Using IS_REACHABLE() to define it avoids the warning.

Fixes: 3785bc17 ("[media] b2c2: break it into common/pci/usb directories")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/common/b2c2/flexcop-fe-tuner.c

@@ -24,8 +24,7 @@
 
 /* Can we use the specified front-end?  Remember that if we are compiled
  * into the kernel we can't call code that's in modules.  */
-#define FE_SUPPORTED(fe) (defined(CONFIG_DVB_##fe) || \
-	(defined(CONFIG_DVB_##fe##_MODULE) && defined(MODULE)))
+#define FE_SUPPORTED(fe) IS_REACHABLE(CONFIG_DVB_ ## fe)
 
 #if FE_SUPPORTED(BCM3510) || (FE_SUPPORTED(CX24120) && FE_SUPPORTED(ISL6421))
 static int flexcop_fe_request_firmware(struct dvb_frontend *fe,