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>

include/net/nfc/hci.h

@@ -83,6 +83,10 @@ struct nfc_hci_pipe {
 };
 
 #define NFC_HCI_MAX_CUSTOM_GATES	50
+/*
+ * According to specification 102 622 chapter 4.4 Pipes,
+ * the pipe identifier is 7 bits long.
+ */
 #define NFC_HCI_MAX_PIPES		127
 struct nfc_hci_init_data {
 	u8 gate_count;