sky2: EEPROM read/write bug fixes

Cleanup and harden the routines accessing the EEPROM.
  1. Prevent spin forever waiting for the TWSI bus
  2. Fix write eeprom to write full words rather than only 16 bits
    Luckly the vendor doesn't provide EEPROM in Linux format so it must never
    have been used.
  3. Don't allow partial eeprom writes, not needed, not safe.

These are non-urgent bug fixes.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>