Fix ON/OFF handling on recent ThinkPad t440p BIOS

This pull request fixes the ON/OFF handling on Lenovo ThinkPad t440p with recent BIOSes. See the second half of the thread in #78 for the complete background.

The general idea is that with recent BIOSes, the call to _DSM does not switch the card OFF, the transition to D3 does. An end result is that the card's identity does not change in lspci and the current state detection method in bbswitch is broken (and the card cannot be turned back ON after it is OFF). This PR implements a different mode where the card state is inferred from the ACPI-reported power state. Set use_acpi_to_detect_card_state to 1 to use this new mode.