gpio: davinci: Use signed type for 'irq' variable

Variable 'irq' is declared as unsigned and then used to store
negative return values from irq_alloc_descs() such as -EINVAL.
This patch fix this by declaring the variable as a signed.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>