hw/net/eepro100: Implement read-only bits in MDI registers

hw/net/eepro100: Implement read-only bits in MDI registers

Although we defined an eepro100_mdi_mask[] array indicating which bits
in the registers are read-only, we weren't actually doing anything with
it. Make the MDI register-write code use it rather than manually making
register 1 read-only and leaving the rest as reads-as-written. (The
special-case handling of register 0 remains as before since its mask is
all-zeros and the special casing happens before we apply the masking.)

Change-Id: I28b6ee4691f5e8c325430eb429bee1b06c9a818e
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1402159924-13853-1-git-send-email-peter.maydell@linaro.org
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>