[ALSA] fix sound/isa/gus/interwave.c compile with PNP=n

AMD InterWave driver
Emmanuel Colbus sent this patch one month ago with the following
description:

There is a trivial bug in the file sound/isa/gus/interwave.c .
The variable isapnp is defined only if CONFIG_PNP is enabled, but it is
always used few lines after.
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/isa/gus/interwave.c

@@ -79,8 +79,10 @@ module_param_array(id, charp, NULL, 0444);
 MODULE_PARM_DESC(id, "ID string for InterWave soundcard.");
 module_param_array(enable, bool, NULL, 0444);
 MODULE_PARM_DESC(enable, "Enable InterWave soundcard.");
+#ifdef CONFIG_PNP
 module_param_array(isapnp, bool, NULL, 0444);
 MODULE_PARM_DESC(isapnp, "ISA PnP detection for specified soundcard.");
+#endif
 module_param_array(port, long, NULL, 0444);
 MODULE_PARM_DESC(port, "Port # for InterWave driver.");
 #ifdef SNDRV_STB