Return the driver-specific software context of
dev.
The softc is automatically allocated and zeroed when the device is
attached.
The softc is also initialized and present when a device is probed,
but is subject to caveats as described in
DEVICE_PROBE(9).
The size of the allocation is determined by the device's
driver_t
information used to define the driver.
The softc typically encapsulates the state of this instance of the
device.
Driver writers are discouraged from using their own softc management
mechanisms.
Driver writers should not copy such mechanisms found in drivers in the
tree that predate this function.
