[media] rtl2832_sdr: remove unneeded spinlock irq flags initialization

There is no need to init flags before calling spin_lock_irqsave().
spin_lock_irqsave is macro which stores value to 'flags'

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>