media: atomisp: cleanup isys_irq headers

Don't need to declare those functions with extern:
	drivers/staging/media/atomisp/pci/css_2401_system/host/isys_irq_private.h:51:35:  warning: function 'isys_irqc_state_dump' with external linkage has definition
	drivers/staging/media/atomisp/pci/css_2401_system/host/isys_irq_private.h:68:35:  warning: function 'isys_irqc_reg_store' with external linkage has definition
	drivers/staging/media/atomisp/pci/css_2401_system/host/isys_irq_private.h:85:39:  warning: function 'isys_irqc_reg_load' with external linkage has definition
	drivers/staging/media/atomisp/pci/css_2401_system/host/isys_irq.c:31:35:  warning: function 'isys_irqc_status_enable' with external linkage has definition

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>