Only define BITS_TO_LONGS if not defined

Only define BITS_TO_LONGS if not defined

This removes this nagging complaint on older kernels:

  CC [M]  /home/mcgrof/compat/compat/compat-2.6.32.o
In file included from include/asm-i386/cpufeature.h:11,
                 from include/asm/cpufeature.h:10,
                 from include/asm/alternative.h:43,
                 from include/asm/atomic.h:4,
                 from /home/mcgrof/compat/include/linux/compat-2.6.24.h:10,
                 from /home/mcgrof/compat/include/linux/compat-2.6.h:15,
                 from <command-line>:0:
/home/mcgrof/compat/include/linux/bitops.h:10:1: warning: "BITS_TO_LONGS" redefined
In file included from include/asm/alternative.h:6,
                 from include/asm/atomic.h:4,
                 from /home/mcgrof/compat/include/linux/compat-2.6.24.h:10,
                 from /home/mcgrof/compat/include/linux/compat-2.6.h:15,
                 from <command-line>:0:
include/linux/types.h:6:1: warning: this is the location of the previous definition
In file included from include/asm-i386/cpufeature.h:11,
                 from include/asm/cpufeature.h:10,
                 from include/asm/alternative.h:43,
                 from include/asm/atomic.h:4,
                 from /home/mcgrof/compat/include/linux/compat-2.6.24.h:10,
                 from /home/mcgrof/compat/include/linux/compat-2.6.h:15,
                 from <command-line>:0:

Note that this means we'll have to keep include/linux/bitops.h up to
date on our tree now. Shouldn't be an issue but just worth noting.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>