modpost: use a table rather than a giant if/else statement.

We look for symbols of form __mod_<busname>_device_table, and for all
but three cases we use a standard interation function (do_table) to
walk over the contents and dump out the aliases.

Alessandro Rubini did this first, I just repainted the bikeshed a bit.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Alessandro Rubini <rubini@gnudd.com>