HID: i2c-hid: acpi: Drop unneded NULL check of adev

The driver is enumerated on ACPI platforms, so adev is valid.
Since there is no valid I²C ID table provided, there is no
possibility to bind a device to this driver via user space.
Hence, drop unneeded NULL check of adev.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>