NFC: protect nci_data_exchange transactions

      Protect 'cb' and 'cb_context' arguments in nci_data_exchange.
In fact, this implements a queue with max length of 1 data
exchange transactions in parallel.

Signed-off-by: Ilan Elias <ilane@ti.com>
Acked-by: Lauro Ramos Venancio <lauro.venancio@openbossa.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>