gpio: aggregator: Drop pre-initialization in get_arg()

In get_arg(), the variable start is pre-initialized, but overwritten
again in the first statement.  Rework the assignment to not rely on
pre-initialization, to make the code easier to read.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200701114212.8520-2-geert+renesas@glider.be
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>