s2io: fix documentation about intr_type

The documentation for intr_type module parameter of the s2io driver is
not consistent with the code. The comments in drivers/net/s2io.c are
OK, but Documentation/networking/s2io.txt is wrong.

Pointed out by Andrew Hecox.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>