Fix bugs in c header

[raffi-g]

Some fixes in the C template:

• I would say, that the reserved bits in the structs were still incorrect after the recent fix.
• The _MASK macro used the wrong attribute (reg.address instead of bf.mask).
• If a description contains multiple lines, it was not handled correctly. (I think that's also the case for some other targets.)

[arnfol]

The problem was fixed in #26