staging: rtl8188eu: remove PWR_CMD_READ

None of the power flows and transitions includes a read command.
PWR_CMD_READ can be removed.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210718173610.894-9-martin@kaiser.cx


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