[En-Nut-Discussion] Improvements to at91_emac.c

[Thiago A. Corrêa]

Hi,

   I don't have any at91 boards here to test this out, so I'm not
commiting directly.
   This patch is a backport from changes I've made to the AVR32 port,
and it's working nicely on the AVR32 tree.
   The At91 and the AVR32 MAC devices are very similar. So much, that
the linux kernel has only one driver for both. I'm not doing as the
linux kernel, because I don't know how we could share the driver, but
still back porting some stuff should be nice :)

   Sumary of changes:
   * Use EMAC_TX_BUFFERS in the TX initialization buffers to make it
behave correctly if that macro changes.
   * Remove EMAC_CAF from the initialization. This flag sets the EMAC
to Promiscuos mode, and all network packages will be copied into
memory, wasting CPU cycles to process and discard.
   * Make PHY ID configurable, as the TODO comment intended.

   Patch is attached, but if it gets blocked, I've also uploaded it to
SF patch tracker.

Kind Regards,
   Thiago A. Correa