staging: r8188eu: simplify if-else statement

Simplify a nested if-else statement to a single if-else statement in
rtw_set_threshold(). This improves readability and allows us to
remove the local variable threshold.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20220329202141.7028-7-straube.linux@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
