wlan: Correct the ini value to enable default sap interface

Currently default value of ini gEnabledefaultSAP is assigned
as the name of the ini, which is resulting in undefined
behaviour.
To resolve this issue, update the default value of ini
gEnabledefaultSAP to empty string.
Additionally add a check to validate the empty string before
open adapter for default sap interface.

Change-Id: Id4391d161cb67a58cc94bbc88ac8dbdfd8c6eb6a
CRs-fixed: 2391821