gpio: stmpe: Move chip registration

Make sure to register the GPIO chip after requesting the
interrupt and setting up the IRQ members of the irqchip.

Fixes: 97450796 ("gpio: stmpe: Use irqchip template")
Reported-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Patrice Chotard <patrice.chotard@st.com>
Link: https://lore.kernel.org/r/20200728072706.348725-1-linus.walleij@linaro.org