Bluetooth: Fix checking for master LTKs

When the rename of STK_SLAVE to simply STK happened we missed this place
in the ltk_type_master function. Now, checking for master is as simple
as checking whether the type is SMP_LTK. The helper function is kept
around for better readability in the (right now three) callers and for
simpler extension with new key types in the future.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Tested-by: Lukasz Rymanowski <lukasz.rymanowski@tieto.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>