platform_get_irq() return negative value on failure, so null check of
irq is incorrect. Fix it by comparing whether it is less than zero.

Fixes: dcc21cc0 ("mfd: Add driver for Atmel Microcontroller on iPaq h3xxx")
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Lv Ruyi <lv.ruyi@zte.com.cn>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220412085305.2533030-1-lv.ruyi@zte.com.cn