arc_emac: fix compile-time errors & warnings on PPC64

As reported by "kbuild test robot" there were some errors and warnings
on attempt to build kernel with "make ARCH=powerpc allmodconfig".

And this patch addresses both errors and warnings.
Below is a list of introduced changes:
1. Fix compile-time errors (misspellings in "dma_unmap_single") on PPC.
2. Use DMA address instead of "skb->data" as a pointer to data buffer.
This fixed warnings on pointer to int conversion on 64-bit systems.
3. Re-implemented initial allocation of Rx buffers in "arc_emac_open" in
the same way they're re-allocated during operation (receiving packets).
So once again DMA address could be used instead of "skb->data".
4. Explicitly use EMAC_BUFFER_SIZE for Rx buffers allocation.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

Cc: netdev@vger.kernel.org
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Francois Romieu <romieu@fr.zoreil.com>
Cc: Joe Perches <joe@perches.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Mischa Jonker <mjonker@synopsys.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: linux-kernel@vger.kernel.org
Cc: devicetree-discuss@lists.ozlabs.org
Cc: Florian Fainelli <florian@openwrt.org>
Cc: David Laight <david.laight@aculab.com>
Signed-off-by: David S. Miller <davem@davemloft.net>