[MIPS] RM: fix EISA=n compilation

This patch fixes the following build error with CONFIG_EISA=n caused by
commit 231a35d3:

<--  snip -->

...
  LD      .tmp_vmlinux1
arch/mips/sni/built-in.o: In function `snirm_a20r_setup_devinit':
a20r.c:(.init.text+0x42c): undefined reference to `sni_eisa_root_init'
a20r.c:(.init.text+0x42c): relocation truncated to fit: R_MIPS_26 against `sni_eisa_root_init'
arch/mips/sni/built-in.o: In function `snirm_setup_devinit':
rm200.c:(.init.text+0x52c): undefined reference to `sni_eisa_root_init'
rm200.c:(.init.text+0x52c): relocation truncated to fit: R_MIPS_26 against `sni_eisa_root_init'
make[1]: *** [.tmp_vmlinux1] Error 1

<--  snip  -->
Signed-off-by: NAdrian Bunk <bunk@kernel.org>
Acked-by: NThomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

include/asm-mips/sni.h

@@ -228,7 +228,14 @@ extern void sni_pcimt_irq_init(void);
 extern void sni_cpu_time_init(void);
 
 /* eisa init for RM200/400 */
+#ifdef CONFIG_EISA
 extern int sni_eisa_root_init(void);
+#else
+static inline int sni_eisa_root_init(void)
+{
+	return 0;
+}
+#endif
 
 /* common irq stuff */
 extern void (*sni_hwint)(void);