staging: r8188eu: support only us in PWR_CMD_DELAY

PWR_CMD_DELAY has only a single remaining user, who needs a delay in us.
Remove the code and defines for delays in ms.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220704145221.159949-3-martin@kaiser.cx


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