bluetooth: ATT: Ignore signed writes on EATT bearer

Core Specification 5.3 Vol 3. Part G. 4.2:
The Signed Write Without Response sub-procedure shall only be supported
on the LE Fixed Channel Unenhanced ATT bearer.

This was affecting GATT/SR/GAW/BI-38-C qualification test.

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>