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>