tifm_sd: handle non-power-of-2 block sizes

It is possible to handle arbitrary block sizes with tifm card reader by
conditionally switching to PIO in case such block has to be delivered. At
the beginning of each request, DMA is either disabled (non-power-of-2 block
size) or set to load time user preference.

Signed-off-by: Alex Dubov <oakad@yahoo.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>