Fix incorrect enum comparison of LE close reason

The LE close reason is tGATT_DISCONN_REASON but it's compared against
tHCI_ERROR_CODE. Besides just being different types, they are also
backed by different primitives (u16 for gatt vs u8 for hci). This fails
in stricter build environments (i.e. ChromeOS).

Bug: 187836537
Tag: #floss
Test: atest --host bluetooth_test_gd
Change-Id: I106c9555e82160f5f66fe24acf933b570ce8b821