Bluetooth: Mesh: Fix resending segments on correct bearer

The GATT bearer is a reliable one so there should theoretically never
be a need to resend segments. If however for some strange reason the
proxy client doesn't immediately ack all segments we should do the
resending on the GATT bearer, rather than sending them over
advertising.

Fixes #17907

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>