NFC: st21nfca: Improve st21nfca initialization by handling reboot properly

Change in st21nfca_hci_platform_init in order to handle in a better way the
internal reboot command.
Once the reboot is completed, the driver expect to receive a 0x7e filled
buffer.

Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>