Use the nfc- rather than phy-device in firmware-management code that
needs a valid struct device.

This specifically fixes a NULL-pointer dereference in
nfcmrvl_fw_dnld_init() during registration when the underlying tty is
one end of a Unix98 pty.

Note that the driver still uses the phy device for any debugging, which
is fine for now.

Fixes: 3194c687

 ("NFC: nfcmrvl: add firmware download support")
Cc: stable <stable@vger.kernel.org>     # 4.4
Cc: Vincent Cuissard <cuissard@marvell.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>