nfc: pn533/usb.c: fix spelling of "functions"

Fix typo/spello of "functions".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-nfc@lists.01.org
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: netdev@vger.kernel.org
Cc: Jiri Kosina <trivial@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>