irqchip/mbigen: Repair non-kernel-doc notation

Fix kernel-doc warnings in irq-mbigen.c:

irq-mbigen.c:29: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * In mbigen vector register
irq-mbigen.c:43: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * offset of clear register in mbigen node
irq-mbigen.c:50: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * offset of interrupt type register

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Jun Ma <majun258@huawei.com>
Cc: Yun Wu <wuyun.wu@huawei.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Aditya Srivastava <yashsri421@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210905033644.15988-1-rdunlap@infradead.org