staging: r8188: remove unused power command

None of the remaining power transition steps used PWR_CMD_READ.
It can be removed.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211219183926.4746-8-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>