staging: rtl8712: Use __packed instead of __attribute__ ((packed))

According to scripts/checkpatch.pl, the __packed macro
from linux/compiler.h should be used.

Signed-off-by: Michael Fiedler <michael.fiedler@informatik.stud.uni-erlangen.de>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>