mmc_spi: put signals to low power off fix

The original intention was to write a zero byte to mmc to force spi
signals to low when doing power off. Somehow the spi_w8r8 call got there
so a read followed the write of single zero byte. This patch changes
that to simple write of zero byte without the following read.
This way the power off is more reliable and completely sufficient.

Signed-off-by: Jan Nikitenko <jan.nikitenko@gmail.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>