iio: event: use short-hand variable in iio_device_{un}register_eventset functions

iio: event: use short-hand variable in iio_device_{un}register_eventset functions

With the recent 'iio_dev_opaque' variable name, these two functions are
looking a bit ugly.

This change uses an 'ev_int' variable for the
iio_device_{un}register_eventset functions to make the code a little easier
to read.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20200921103156.194748-1-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>