wifi(implementation): Turn on DFS flag for STA

DFS flag needs to be turned on in the legacy HAL for STA operations. So,
initialize this flag on creation of STA iface.
PS: Not adding an explicit HIDL API for this since there is no need to
toggle this from framework and since the HIDL API's are frozen now.

Bug: 36169790
Test: Verified the fix with regression tests: b/37528903.
Change-Id: I214097fcb399cef7fc0cbd2c278622fac259a3e3