firmware: card_emu: fix 7816-3 10.3.2 0=256 case
add_tpdu_byte() accumulates bytes received from the reader, but passes
reader_to_card = 0, but ISO 7816-3 10.3.2 says P3 == 0 means 256 not 0
Only reachable with P3 == 0 in a receive data phase, but
simtrace2-cardem-pcsc only requests PB_AND_RX when there is command data.
Fix this anyway.
Change-Id: I0fa4741bc1293549816595e6b4e8af9e22bcfbc1
