lib/crc32.c: remove unnecessary __constant

Use cpu_to_le32 instead of __constant_cpu_to_le32.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>