ssb: Add support for block-I/O

This adds support for block based I/O to SSB.
This is needed in order to efficiently support PIO data
transfers to the card.
The block-I/O support is only compiled, if it's selected by the
weird driver that needs it. So there's no overhead for sane devices.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>