m68k: atari: Annotate dummy read in ROM port IO code as __maybe_unused

m68k: atari: Annotate dummy read in ROM port IO code as __maybe_unused

The Atari ROM port IO code uses dummy variables to implement writes
(not supported by the hardware) as reads that encode the write data
in part of the address. The value read from the ROM port in this
operation is discarded.

Annotate dummy variables as __maybe_unused to avoid a compiler warning
with W=1.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
Link: https://lore.kernel.org/r/1590878719-21219-1-git-send-email-schmitzmic@gmail.com
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>