Add __attribute__((aligned(4))) to __packed Tx/Rx descriptors.

This makes gcc generate much more efficient code on architectures
with strong alignment constraints (like sparc64).
We use __packed to tell the compiler to not insert padding between
fields but the start of the descriptors is always 32-bit aligned.
When __packed is used, gcc assumes worst case scenario and generates
complicated code to prevent unaligned accesses.

Inspired by a similar change to ath9k.
Tested on sparc64.

For the record, example to set a field to 1 on a sparc64:
without __attribute__((aligned(4))):
      ldub    [%g2], %g1
      and     %g1, 0, %g1
      stb     %g1, [%g2]
      ldub    [%g2+1], %g1
      and     %g1, 0, %g1
      stb     %g1, [%g2+1]
      ldub    [%g2+2], %g1
      and     %g1, 0, %g1
      stb     %g1, [%g2+2]
      ldub    [%g2+3], %g1
      and     %g1, 0, %g1
      or      %g1, 1, %g1
      stb     %g1, [%g2+3]

with __attribute__((aligned(4))):
      mov     1, %g1
      st      %g1, [%g2]

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