This line is weird in multiple ways.

(CONFIG_GENERIC_CSUM) might be a typo of $(CONFIG_GENERIC_CSUM).

Even if you add '$' to it, $(CONFIG_GENERIC_CSUM) is never evaluated
to 'y' because scripts/Makefile.asm-generic does not include
include/config/auto.conf. So, the asm-generic wrapper of checksum.h
is never generated.

Even if you manage to generate it, it is never included by anyone
because MIPS has the checkin header with the same file name:

  arch/mips/include/asm/checksum.h

As you see in the top Makefile, the checkin headers are included before
generated ones.

  LINUXINCLUDE    := \
                  -I$(srctree)/arch/$(SRCARCH)/include \
                  -I$(objtree)/arch/$(SRCARCH)/include/generated \
                  ...

Commit 4e0748f5 ("MIPS: Use generic checksum functions for MIPS R6")
already added the asm-generic fallback code in the checkin header:

  #ifdef CONFIG_GENERIC_CSUM
  #include <asm/generic/checksum.h>
  #else
    ...
  #endif
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: James Hogan <jhogan@kernel.org>
Cc: linux-kernel@vger.kernel.org