fixed size error when creating dump files of mfclassic

[ricardojrdez]

When reading mifare classic cards, dump files were created of 4K size regardless the card type. Fix in line 689 solves this issue, creating a dump file of the same size than the card being dumped.

[doegox]

Thanks for spotting hardcoded size.
I cleaned the code in c5e8adc
Note that regarding the "fixed size error when creating dump files", your proposed change won't change anything as line 689 (now at https://github.com/nfc-tools/libnfc/blob/master/utils/nfc-mfclassic.c#L712) is only about erasing the buffer. Real size of written dump is defined at https://github.com/nfc-tools/libnfc/blob/master/utils/nfc-mfclassic.c#L742

I tried on MFC Mini / MFC 1k / MFC 4k / MFP 2k, all dump sizes seem ok now.