ofnet: move the allocation of the transmit buffer into a function

ofnet: move the allocation of the transmit buffer into a function

In the current code search_net_devices() uses the "alloc-mem" command
from the IEEE1275 User Interface for allocation of the transmit buffer
for the case when GRUB_IEEE1275_FLAG_VIRT_TO_REAL_BROKEN is set.

I don't have hardware where this flag is set to verify if this
workaround is still needed. However, further changes to ofnet will
require to execute this workaround one more time. Therefore, to
avoid possible duplication of code I'm moving this piece of
code into a function.

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>