libertas: correct packing of rxpd structure

Older Gcc compilers (3.4.5 tested) need additional hints in order to get
the packing of the rxpd structure (which contains a 16 bit union)
correct on the ARM processor.

struct txpd does not need these hints since it contains a 32 bit union
that packs naturally.

Signed-off-by: R.J.Dunlop <rdunlop@guralp.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>