Fix remaining warnings from Sparse (void return)

Fix remaining warnings from Sparse (void return)

Sparse report:

hw/display/vga.c:2000:5: warning: returning void-valued expression
hw/intc/arm_gic.c:707:9: warning: returning void-valued expression
hw/intc/etraxfs_pic.c:138:9: warning: returning void-valued expression
hw/nvram/fw_cfg.c:475:5: warning: returning void-valued expression
hw/timer/a9gtimer.c:124:5: warning: returning void-valued expression
hw/tpm/tpm_tis.c:794:5: warning: returning void-valued expression
hw/usb/hcd-musb.c:558:9: warning: returning void-valued expression
hw/usb/hcd-musb.c:776:13: warning: returning void-valued expression
hw/usb/hcd-musb.c:867:5: warning: returning void-valued expression
hw/usb/hcd-musb.c:932:5: warning: returning void-valued expression
include/qom/cpu.h:584:5: warning: returning void-valued expression
monitor.c:4686:13: warning: returning void-valued expression
monitor.c:4690:13: warning: returning void-valued expression

Cc: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Andreas Färber <afaerber@suse.de>
Cc: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>