SMC91111: Fix compilation warnings

This patch fixes the following warning messages coming out of
'drivers/net/smc91111.h' when compiled for 'vexpress_aemv8a':

warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

Currently this issue seems to surface when SMSC is compiled for 64-bit
ARMv8 platforms, so the change is protected under CONFIG_ARM64, so that
it doesn't break other existing platforms.

In addition this patch tries to fix some checkpatch errors and warnings
(others related to camel-casing and volatile usage will be addressed
by a later patch).

This fix has been tested on both ARMv8 foundation model v1 and v2.

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@freescale.com>