I got this issuse too, but yestedey find solution. This problem probably with kernel module (radio-bcm2048.ko)
You need download it (attached) - this version i copied from my wife's N900 (my phone using Kernel Power-53, wife's choice CSSU-Thumb). After download it, follow these instruction:

1. Unpack archive radio-bcm2048.ko.gz
2. Copy radio-bcm2048.ko to N900 internal memory over USB and unplug N900 from PC
3. Go to Terminal on N900 and get root ("sudo gainroot" or "root")
-. Try load FM-module: modprobe /lib/modules/current/radio-bcm2048.ko (you get error, ignore it)
4. Backup existing module: "cp /lib/modules/current/radio-bcm2048.ko /lib/modules/current/radio-bcm2048.ko.bak"
5. Move working module: "mv /home/user/MyDocs/radio-bcm2048.ko /lib/modules/current/radio-bcm2048.ko"
-. Try load FM-module: modprobe /lib/modules/current/radio-bcm2048.ko (you get error again, ignore it)
6. Reboot and try FM radio.

My FM working with it; i don't sure with "-" strings need, but it worked for me.

PS. Sorry, my English awful (i speel Russian, i learn English by seeing films, Google translate and technical manuals). Hope i help you.