The "specific" lines configuration would not work due to index error in
the callback configuration. While fixing the error, simplifying the API
by moving the afio calls to the calling context seemed a cleaner way to
do things
A while back, macros had to be put in place to avoid letting the
compiler directly use the bitfields. This was necessary because the
compiler used strb instruction which only write bytes. On the AHB bus,
byte writes are transformed into word writes by repeating the byte,
which caused mayhem in the registers. After a lot of research, turns out
the packed attribute stops the compiler from does optimal (word) writes
and isn't needed anyway. Removing them fixes the issue
