Implementation of card_ctl function with SC_CARDCTL_GET_SERIALNR

Implementation of card_ctl function with SC_CARDCTL_GET_SERIALNR
indenting and lenghth limits fixed. 
About checking the content: do you mean checking if it appears to be a CNS serial number according to specification, or simply check the length? The buffer (mem)copied is 16 bytes long anyway, so no risks of buffer overflows, I think ...