module: fix broken isapnp handling in file2alias

Handling of isapnp module aliases was broken by commit
626596e2 by changing "isapnp" string to "isa".
The code was then modified by commit
e49ce141 but this bug remained.

Change the string back to "isapnp".

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>