Remove obsolete Prism54.org driver support (driver_prism54.c)

Remove obsolete Prism54.org driver support (driver_prism54.c)

The Prism54.org project seems have been dead for a while and it does not
look like this driver would ever be maintained again. Furthermore, it is
difficult to find a version that would work with the driver_prism54.c
wrapper and there is another driver for these card in the Linux kernel
tree.

The hostapd integration in driver_prism54.c is quite different from the
other driver wrappers and would require major effort to get it cleaned
up. Since there does not seem to be any real users for the cleaned up
version, there does not seem to be justification to spend this effort on
the wrapper. This old code is making it much more difficult to clean up
the driver interface and at this point, the best option seems to be to
remove the driver wrappers. Should someone really still need this, the
old code will continue to be available in hostapd 0.6.x.