[MIPS] Add empty argument parenthesis to GCC_IMM_ASM

This is to clarify that GCC_IMM_ASM does not take an argument as the
context of the macro's invocation seems to imply.

As suggested by Maciej W. Rozycki (macro@linux-mips.org).

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>