Bluetooth: Add a convenience function to check for SSP enabled

It's a very common test to see if both the local and the remote device
have SSP enabled. By creating a simple function to test this we can
shorten many if-statements in the code.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>