nfc: nci: Add comment to explain NCI_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>