net: tsec: Use portable types and accessors for BDs

Currently, the buffer descriptor (BD) fields cannot be
correctly accessed by a little endian processor.  This
patch fixes the issue by making the access of BDs to be
portable among different cpu architectures.

Use portable data types for the Rx/Tx buffer descriptor
fields.  Use portable I/O accessors to insure that the
big endian BDs are correctly accessed by little endian
cpus too, and to insure proper sync with the H/W.
Removed the redundant RTXBD "volatile" type, as proper
synchronization around BD data accesses is provided by
the I/O accessors now.
The "sparse" tool was also used to verify the correctness
of these changes.

Cc: Scott Wood <scottwood@freescale.com>

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>