iio: fix memory leak in iio_device_register_eventset()

In the Linux kernel, the following vulnerability has been resolved:

iio: fix memory leak in iio_device_register_eventset()

When iio_device_register_sysfs_group() returns failed,

iio_device_register_eventset() needs to free attrs array.

Otherwise, kmemleak would scan & report memory leak as below:

unreferenced object 0xffff88810a1cc3c0 (size 32):

comm "100-i2c-vcnl302", pid 728, jiffies 4295052307 (age 156.027s)

backtrace:

__kmalloc+0x46/0x1b0

iio_device_register_eventset at drivers/iio/industrialio-event.c:541

__iio_device_register at drivers/iio/industrialio-core.c:1959

__devm_iio_device_register at drivers/iio/industrialio-core.c:2040