Fix the following cppcheck warnings:
[libnfc/drivers/pn532_uart.c:492]: (style) The scope of the variable 'res' can be reduced
[libnfc/chips/pn53x.c:320]: (style) The scope of the variable 'res' can be reduced
[libnfc/chips/pn53x.c:616]: (style) The scope of the variable 'res' can be reduced
[libnfc/nfc.c:356]: (style) The scope of the variable '_device_found' can be reduced
[utils/nfc-mfclassic.c:178]: (style) The scope of the variable 'key_index' can be reduced