staging: rtl8188eu: use compound assignment operators

Use compound assignment operators to simplify the code and clear
missing spaces around operators checkpatch issues.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20200629161255.28371-1-straube.linux@gmail.com


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