As seen on GCC 4.6 Linaro compiler, control_req buffer is not aligned
on 4 byte boundaray causing data aborts in eth_setup -> conf_buf
during dhcp boot over usb_ether. Fix the issue my aligning control_req
buffer using DEFINE_CACHE_ALIGN_BUFFER.

Tested on am335x_evm platform (beaglebone).
Applies on 2013.10-rc1 branch.

Cc: Tom Rini <trini@ti.com>
Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Joel Fernandes <joelf@ti.com>