carl9170: fix struct alignment conflict

Multiple structures in the carl9170 driver have alignment
impossible alignment constraints that gcc warns about when
building with 'make W=1':

include/shared/fwcmd.h:243:2: warning: alignment 1 of 'union <anonymous>' is less than 4 [-Wpacked-not-aligned]
include/shared/wlan.h:373:1: warning: alignment 1 of 'struct ar9170_rx_frame_single' is less than 2 [-Wpacked-not-aligned]

In the carl9170_cmd structure, multiple members that have an explicit
alignment requirement of four bytes are added into a union with explicit
byte alignment, but this in turn is part of a structure that also has
four-byte alignment.

In the wlan.h header, multiple structures contain a ieee80211_hdr member
that is required to be two-byte aligned to avoid alignmnet faults when
processing network headers, but all members are forced to be byte-aligned
using the __packed tag at the end of the struct definition.

In both cases, leaving out the packing does not change the internal
layout of the structure but changes the alignment constraint of the
structure itself.

Change all affected structures to only apply packing where it does
not violate the alignment requirement of the contained structure.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>