net: fec_mxc: Fix timeouts during tftp transfer

Performing tftp transfers on mx28 results in random timeouts.

Hector Palacios and Robert Hodaszi analyzed the root cause being related to the
wrong alignment of the 'buff' buffer inside fec_recv().

Benoît Thébaudeau provided an excellent analysis of the alignment bug that is
present on older versions, such as GCC 4.5.4:

http://marc.info/?l=u-boot&m=137942904906131&w=2



Use ALLOC_CACHE_ALIGN_BUFFER() to avoid alignment issues from older GCC
versions.

Reported-by: Hector Palacios <hector.palacios@digi.com>
Tested-by: Oliver Metz <oliver@freetz.org>
Tested-by: Hector Palacios <hector.palacios@digi.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Tested-by: Marek Vasut <marex@denx.de>