m32r: fix build warning

m32r: fix build warning

Some m32r builds were having a warning:

  arch/m32r/include/asm/cmpxchg.h:191:3: warning: value computed is not used
  arch/m32r/include/asm/cmpxchg.h:68:3: warning: value computed is not used

Taking the idea from commit 76be40389b10 ("ARM: cmpxchg: avoid warnings
from macro-ized cmpxchg() implementations") the m32r implementation is
changed to use a similar construct with a compound expression instead of
a typecast, which causes the compiler to not complain about an unused
result.

Link: http://lkml.kernel.org/r/1484432664-7015-1-git-send-email-sudipm.mukherjee@gmail.com
Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>