ethernet: ucc_geth: simplify rx/tx allocations

Since kmalloc() is nowadays [1] guaranteed to return naturally
aligned (i.e., aligned to the size itself) memory for power-of-2
sizes, we don't need to over-allocate the align amount, compute an
aligned address within the allocation, and (for later freeing) also
storing the original pointer [2].

Instead, just round up the length we want to allocate to the alignment
requirements, then round that up to the next power of 2. In theory,
this could allocate up to about twice as much memory as we needed.  In
practice, (a) kmalloc() would in most cases anyway return a
power-of-2-sized allocation and (b) with the default values of the
bdRingLen[RT]x fields, the length is already itself a power of 2
greater than the alignment.

So we actually end up saving memory compared to the current
situtation (e.g. for tx, we currently allocate 128+32 bytes, which
kmalloc() likely rounds up to 192 or 256; with this patch, we just
allocate 128 bytes.) Also struct ucc_geth_private becomes a little
smaller.

[1] 59bb4798 ("mm, sl[aou]b: guarantee natural alignment for
kmalloc(power-of-two)")

[2] That storing was anyway done in a u32, which works on 32 bit
machines, but is not very elegant and certainly makes a reader of the
code pause for a while.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>