ucc_geth: separate out rx/tx ring alloc and free operations

Factor out the the existing allocation and free operations
so that they can be used individually.

This is to improve code readability, and also to prepare for
possible future changes like better error recovery and more
dynamic configuration (e.g on-the-fly resizing of the rings).

This change represents a straight up relocation of the existing
code into separate routines without changing any of the contained
code itself.  Local variables are relocated as necessary.
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>