net: pcnet: Drop PCNET_HAS_PROM

All of one PCNET users has this option set, make this default
and drop this config option.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>