nfc: st21nfca: Remove unnecessary cast

In st21nfca_connectivity_event_received(), the return value of
devm_kzalloc() is unnecessarily cast from void*. Remove cast.

Issue identified with Coccinelle.

Signed-off-by: Alex Dewar <alex.dewar90@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>