The file in SRC_URI returns 404...
All prism54 firmwares are stored here: http://daemonizer.de/prism54/prism54-fw/
take a look at bug #259333
please hardmask net-wireless/prism54-firmware-1.0.4.3 since - src_uri changed - homepage changed and - version is too old for current kernels, so the ebuild is really useless and shouldn't be marked as stable.
i fixed the SRC_URI in cvs. still looking for a way to clean up this mess though... thanks for the report - sorry for the delay.