ACS ACR122U is an USB NFC reader, PC/SC and CCID compilant, based
on NXP PN532 chip.

Internally, it's build of MCU, PN532 and an antenna. MCU makes the
device CCID and PC/SC compilant and provide USB connection.

In this achitecture, a host cannot talk directly to PN532 and must
rely on MCU. Luckily, MCU exposes pseud-APDU through PC/SC Escape
mechanism which let the host to transmit standard PN532 commands
directly to PN532 chip with some limitations.

The frame roughly looks like:

    CCID header    |          APDU header           |    PN532 header
(pc_to_rdr_escape) |  (pseudo apdu Direct Tramsmit) | (len, TFI, cmd, params)

Accordign to limitations, ACR122U does't provide any mechanism to
abort last issued command.
Signed-off-by: NWaldemar Rymarkiewicz <waldemar.rymarkiewicz@tieto.com>
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>