CodingStyle: Relax restrictions on types of bit-fields.

C99 only requires compilers to support four types for bit-fields: signed
int, unsigned int, int, and _Bool.  "int" should not be used because it
is implementation-defined whether it is signed.  In practice, we have found
that compilers (in particular, GCC, Clang, and MSVC 2013) support any
integer type.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>