NFC: nfcmrvl_uart: add missing tty-device sanity check

Make sure to check the tty-device pointer before trying to access the
parent device to avoid dereferencing a NULL-pointer when the tty is one
end of a Unix98 pty.

Fixes: e097dc62 ("NFC: nfcmrvl: add UART driver")
Cc: stable <stable@vger.kernel.org>     # 4.2
Cc: Vincent Cuissard <cuissard@marvell.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>