ARM: OMAP2: use 'int' instead of 'unsigned' for variable 'gpmc_irq_start'

'gpmc_irq_start' is mostly used as 'int', and for a variable, do not
suggest to only use 'unsigned' as its type, so use 'int' instead of
'unsigned' for variable 'gpmc_irq_start'.

Also it will fix the related issue (dummy the real world failure):

arch/arm/mach-omap2/gpmc.c:728:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>