libertas_spi: Use workqueue in hw_host_to_card

Use workqueue to perform SPI xfers, it's necessary to fix
nasty "BUG: scheduling while atomic", because
spu_write() calls spi_sync() and spi_sync() may sleep, but
hw_host_to_card() callback can be called from atomic context.
Remove kthread completely, workqueue now does its job.
Restore intermediate buffers which were removed in commit
86c34fe8 that introduced
mentioned bug.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>