mmc: sh-mmcif: fix a race, causing an Oops on SMP

Oopses have been observed on SMP in the sh-mmcif IRQ thread, when the two
IRQ threads run simultaneously on two CPUs. Also take care to guard the
timeout work and the DMA completion callback from possible NULL-pointer
dereferences and races.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>