staging: r8188eu: remove the global DriverState

staging: r8188eu: remove the global DriverState

DriverState in struct adapter is never set. It remains 0 (DRIVER_NORMAL)
all the time, DriverState != DRIVER_DISAPPEAR is always true.

Both DriverState and the check can be removed. enum driver_state contains
the possible values of DriverState. This enum can be removed as well.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220206161809.423031-10-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>