staging: r8188eu: clean up the usb_readXY functions

Remove unnecessary variables, summarize declarations and assignments.

Acked-by: Phillip Potter <phil@philpotter.co.uk>
Acked-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210821164859.4351-6-martin@kaiser.cx


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