MIPS: fix vdso different address spaces

sparse report build warning as follows:
arch/mips/vdso/vdso-n32-image.c:13:35:
incorrect type in assignment (different address spaces) @@
expected void *[usertype] vdso @@     got void [noderef] <asn:1> * @@

Reported-by: kernel test robot <lkp@intel.com>

Signed-off-by: Sunguoyun <sunguoyun@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>