staging: vchiq_2835_arm: quit using custom down_interruptible()

vchi_killable.h overrides down_interruptible() by implementing a
function similar to down_killable(). To make things simpler we turn
calls to down_interruptible() into kernel's implementation of
down_killable().

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>