If you convert the AP1252 to LAP1252 (autonomous to LWAPP conversion) then you should have no problem running it on the 2125.
However, you would never been able to achieve full potential of the AP...
From the 2100 Series Q&A:
Q. What are the exact differences between the Cisco 2100 Series Wireless LAN Controller and the Cisco 4400 Series Wireless LAN Controllers?
A. The Cisco 2100 Series Wireless LAN controllers are designed to provide basic, secure coverage, while Cisco 4400 Series controllers provide flagship performance and scale that is needed for campus and medium and large branch offices. While the Cisco 2100 Series supports transaction-oriented wireless environments, the 4400 Series is recommended for high-data-rate and multicast-intensive applications such as large data files, video, push-to-talk.
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Basically, 2100's have 8 ethernet (100Mb) ports. No link aggregation, no Gigabit ports. The most bandwidth that could ever get to an AP on a 2100 would be 100Mb theoretically, as the controller does not have a port capable of more than that.
The 4402 has 2 gigabit ports (4404 has 4), which can be aggregated into a 2gig (4gig) etherchannel if your network supports it. In otherwords, you can achieve a lot more aggregate throughput on a 4400 Series.
Remember, the 1252 is an 802.11N AP capable of like ~300Mb to a client...
