uqmi: introduce devpath option

uqmi: introduce devpath option

Introduce the devpath option to find the control channel device from a
hardware path for a USB or a WWAN device.

This option is useful when there are multiple modems connected to the
system. The name of the control channel device of a modem can change
depending on which modem initialises first or if it was recently plugged
in. The devpath option allows specifying the hardware path of the modem
where the control channel device will be found using that.

For the USB device hardware path, it is allowed to specify the USB port
number the modem is directly connected to.

If the device and devpath options are both set, devpath takes precedence
over device.

The USB device hardware path of a control channel device can be found by:

readlink -f /sys/class/usbmisc/cdc-wdmX/device

The WWAN device hardware path of a control channel device can be found by:

readlink -f /sys/class/wwan/wwanXqmiX/device

An example uci configuration would be:

config interface 'wwan_usb1'
	option proto 'qmi'
	option auth 'none'
	option devpath '/sys/devices/platform/1e1c0000.xhci/usb1/1-1'
	option apn 'internet'
	option pdptype 'ipv4v6'

Or:

config interface 'wwan_pcie1'
	option proto 'qmi'
	option auth 'none'
	option devpath '/sys/devices/platform/soc/11280000.pcie/pci0003:00/0003:00:00.0/0003:01:00.0'
	option apn 'internet'
	option pdptype 'ipv4v6'

Signed-off-by: Chester A. Unal <chester.a.unal@arinc9.com>