include/linux/bitfield.h: remove 32bit from FIELD_GET comment block

I do not see anything that restricts this macro to 32 bit width.

Link: http://lkml.kernel.org/r/1505921975-23379-1-git-send-email-yamada.masahiro@socionext.com
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>