[ARM] 3480/1: ixp4xx: fix irq2gpio array type

Patch from Lennert Buytenhek

The irq2gpio array was recently converted from an array of ints to an
array of chars (by patch 3368/1.)  However, this array contains elements
that are -1, and on ARM, the char type is unsigned by default, so this
patch broke the GPIO check in ixp4xx_set_irq_type.

Change the 'char' to be a 'signed char' to fix this.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Deepak Saxena <dsaxena@plexity.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>