mmc_spi: Fix mmc-over-spi regression

Patch 49dce689

 changed the sysfs data
structures for SPI in a way which broke the MMC-over-SPI host driver.

This patch fixes that regression by changing the scheme used to keep
from knowingly trying to use a shared bus segment, and updates the
adjacent comments slightly to better explain the issue.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>