drivers/rtc/rtc-jz4740.c: fix IRQ error check

The irq field of the jz4740_irc struct is unsigned.  Yet we assign the
result of platform_get_irq() to it.  platform_get_irq() may return a
negative error code and the code checks for this condition by checking if
'irq' is less than zero.  But since 'irq' is unsigned this test will
always be false.  Fix it by making 'irq' signed.

The issue was found using the following coccinelle semantic patch:

//<smpl>
@@
type T;
unsigned T i;
@@
(
*i < 0
|
*i >= 0
)
//</smpl>

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>