net: wwan: t7xx: Add sysfs attribute for device state machine

Add support for userspace to get/set the device mode, device's state
machine changes between (unknown/ready/reset/fastboot).

Get the device state mode:
 - 'cat /sys/bus/pci/devices/${bdf}/t7xx_mode'

Set the device state mode:
 - reset(cold reset): 'echo reset > /sys/bus/pci/devices/${bdf}/t7xx_mode'
 - fastboot: 'echo fastboot_switching > /sys/bus/pci/devices/${bdf}/t7xx_mode'
Reload driver to get the new device state after setting operation.

Signed-off-by: Jinjian Song <jinjian.song@fibocom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>