media: usbvision: remove time_in_irq

Time "in interrupt" accounting with the help of `jiffies' is a pointless
exercise. This variable isn't even used.
Remove time_in_irq.

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>