Clean up value_struct_elt_bitpos

Clean up value_struct_elt_bitpos

value_struct_elt_bitpos is weird: it takes an in/out value parameter,
and it takes an error string parameter.  However, it only has a single
caller, which never uses the "out" value.

I think it was done this way to mimic value_struct_elt.  However,
value_struct_elt is pretty ugly and I don't think it's worth
imitating.

This patch cleans up value_struct_elt_bitpos a bit.

Approved-By: Simon Marchi <simon.marchi@efficios.com>