usb: musb: fix a build error on mips

On mips, we got:

drivers/usb/musb/musb_io.h:44: error: conflicting types for 'readsl'
arch/mips/include/asm/io.h:529: error: previous definition of 'readsl' was here
drivers/usb/musb/musb_io.h:46: error: conflicting types for 'readsw'
arch/mips/include/asm/io.h:528: error: previous definition of 'readsw' was here
drivers/usb/musb/musb_io.h:48: error: conflicting types for 'readsb'

so, should add !defined(CONFIG_MIPS) too.

Cc: Felipe Balbi <balbi@ti.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/musb/musb_io.h

@@ -39,7 +39,8 @@
 
 #if !defined(CONFIG_ARM) && !defined(CONFIG_SUPERH) \
 	&& !defined(CONFIG_AVR32) && !defined(CONFIG_PPC32) \
-	&& !defined(CONFIG_PPC64) && !defined(CONFIG_BLACKFIN)
+	&& !defined(CONFIG_PPC64) && !defined(CONFIG_BLACKFIN) \
+	&& !defined(CONFIG_MIPS)
 static inline void readsl(const void __iomem *addr, void *buf, int len)
 	{ insl((unsigned long)addr, buf, len); }
 static inline void readsw(const void __iomem *addr, void *buf, int len)