made sc_apdu_t.data 'const' (which it used to be)

fixes a lot of warnings which pass a const buffer to the APDU's data

Note that a non-const data member is only required for sc_allocate_apdu
sc_free_apdu. They are currently used with an explicit typecast.
However, sc_allocate_apdu and sc_free_apdu both are not used once in the
entire project. One might also simply throw both functions away.