pcnet: Move BCR defines to header

This moves BCR defines to the common header and immediately makes use of
them to add BCR_APROMWE, replacing the open-coded write check in
pcnet_aprom_writeb.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>