msm: wlan: db.txt: Set min of Tx power of AP/STA mode

Update US reg domain to pick the min of Tx power limit for db.txt
in case the entries are different for STA and SAP. So picking
23(AP) over 24(STA).

Change-Id: Id3be427e69b6e20a6a84f4f8291e4dbf53dea045
CRs-Fixed: 630538
Signed-off-by: Manjunathappa Prakash <prakashpm@codeaurora.org>