treat BTA_BUSY as success to avoid unpair the BLE device

when switch from peripheral mode, both server and client will
call btif_gatt_check_encrypted_link, the first call will start the
encryption. the second call will get callback with BTA_BUSY status,
before encryption complete, which cause unpair with the remote device.
we shouldnot treat BTA_BUSY as error.

bug:13417692
Change-Id: I065432dde2553e8ccee3a14a7b321b85bca8d3ba