libertas: if_sdio: fix buffer alignment in struct if_sdio_card

The commit 886275ce

 (param: lock
if_sdio's lbs_helper_name and lbs_fw_name against sysfs changes)
introduced new fields into the if_sdio_card structure. It caused
missalignment of the if_sdio_card.buffer field and failure at driver
load time:

  ~# modprobe libertas_sdio
  [   62.315124] libertas_sdio: Libertas SDIO driver
  [   62.319976] libertas_sdio: Copyright Pierre Ossman
  [   63.020629] DMA misaligned error with device 48
  [   63.025207] mmci-omap-hs mmci-omap-hs.1: unexpected dma status 800
  [   66.005035] libertas: command 0x0003 timed out
  [   66.009826] libertas: Timeout submitting command 0x0003
  [   66.016296] libertas: PREP_CMD: command 0x0003 failed: -110

Adding explicit alignment attribute for the if_sdio_card.buffer field
fixes this problem.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>