MIPS: Fix clean of vmlinuz.{32,ecoff,bin,srec}

Make doesn't expand shell style "vmlinuz.{32,ecoff,bin,srec}" to the 4
separate files, so none of these files get cleaned up by make clean.
List the files separately instead.

Fixes: ec335292 ("MIPS: Remove all generated vmlinuz* files on "make clean"")
Signed-off-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18491/