Fix SYSCALL_ALIAS for older MIPS assembler

Older MIPS assembler don't support .set for defining aliases.
Using = works for old and new assembers.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>