drivers/nble: Defer GATT long write errors to execute response

Defer Prepare write errors to be sent in Execute write response
Per Bluetooth Spec. The Attribute Value validation is done when an
Execute Write Request is received. Hence, any Invalid Offset or
Invalid Attribute Value Length errors are generated when an
Execute Write Request is received. Fixes test case
TC_GAW_SR_BI_33_C (run with shell).

Fixes: ZEP-218

Change-Id: I3dc4583f519e0da2e1f741c9b532d6a6c1970225
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>