Use void instead of uchar in the endian-swapping function parameters

This allows us to pass pointers to storage that is not an array of
uchar, which it hardly ever is.

Change-Id: Ifea6e497f11a461db432ffff14490d2c2df21906
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>