If a driver's IFDI_ATTACH_PRE function fails, the iflib_device_register
function will free the ctx pointer. However, it does not reset the
device softc pointer to NULL.
This will result in memory corruption as a future access to the now
invalid pointer will corrupt memory that is later allocated on top of
the same memory location.
The iflib_device_deregister function correctly resets the softc pointer
by using device_set_softc().
This clears up the invalid dangling pointer and prevents memory
corruption that could lead to a panic or undefined behavior if the
device's driver failed to attach.
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>