media: imon: use DEVICE_ATTR_RW() helper macro

Use DEVICE_ATTR_RW() helper macro instead of DEVICE_ATTR(), which is
simpler and more readable.

Due to the names of the read and write functions of the sysfs attribute is
normalized, there is a natural association.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>