C99 restricts bitfields type is signed and unsigned integer or _Bool.
See section 6.7.2.1 Structure and union specifiers.
"A bit-field shall have a type that is a qualified or unqualified version
 of _Bool, signed int, unsigned int, or some other implementation-defined type."

Since the compiler packs bitfields anyways, actual data layout in the structure is unaffected.
Signed-off-by: Alex Nemirovsky <alex.nemirovsky@cortina-access.com>