eatt: Fix eatt device duplicate

The eatt_device can already exist when the supported features callback
is called. We should first check for the existing device to avoid adding
another one with the same address.

Bug: 263408289
Tag: #feature
Test: atest --host net_test_eatt --no-bazel-mode
Change-Id: I614d1b258ffed6554ea52da78815f871946bb86f