usb: typec: stusb160x: fix signedness comparison issue with enum variables

usb: typec: stusb160x: fix signedness comparison issue with enum variables

chip->port_type and chip->pwr_opmode are enums and when GCC considers them
as unsigned, the conditions are never met.
This patch takes advantage of the ret variable and fixes the following
warnings:
drivers/usb/typec/stusb160x.c:548 stusb160x_get_fw_caps() warn: unsigned 'chip->port_type' is never less than zero.
drivers/usb/typec/stusb160x.c:570 stusb160x_get_fw_caps() warn: unsigned 'chip->pwr_opmode' is never less than zero.

Fixes: da0cb6310094 ("usb: typec: add support for STUSB160x Type-C controller family")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Link: https://lore.kernel.org/r/20201028163309.12878-1-amelie.delaunay@st.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>