nfc: hci: Add comment to explain NFC_HCI_MAX_PIPES

According to specification etsi 102 622 chapter 4.4 pipes identifier
is 7 bits long giving a 127 possible pipes value.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>