NFC: st21nfcb: Fix "NULL pointer dereference" possible error

When the platform with CONFIG_ST21NFCB_I2C=y without any st21nfcb component
physically connected a:
"Unable to handle kernel NULL pointer dereference at virtual address" may
show up at driver initialization phase.

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