mbox_unregister_vf_irq and mbox_unregister_pf_irq returns void value.
mbox_unregister_irq also returns void.
Clang with flags '-Wall -Wextra -pedantic' complains about:
void function should not return void expression
Signed-off-by: Krzysztof Kanas <kkanas@marvell.com>