sd: sdhci: Delete over-zealous power check

This check was conditionalising SD card operation on the card being
powered by the SDHCI host controller. It is however possible
(particularly in embedded systems) for the power control of the SD card
to be managed outside of SDHCI. This can be as trivial as hard-wiring
the SD slot VCC to a constant power-rail.

This means the guest SDHCI can validly opt-out of the SDHCI power
control feature while still using the card. So delete this check to
allow operation of the card with SDHCI power control.

This is needed for at least Xilinx Zynq and Raspberry Pi, and
also makes Freescale i.MX25 work for me. The digilent Zybo board
has a public schematic which shows SD VCC hardwiring:

http://digilentinc.com/Data/Products/ZYBO/ZYBO_sch_VB.3.pdf
bottom of page 3.

Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Reviewed-by: Sai Pavan Boddu <saipava@xilinx.com>
Signed-off-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Message-id: 1450738069-18664-2-git-send-email-Andrew.Baumann@microsoft.com
[AB: Add Pi to list of devices fixed in commit message]
Signed-off-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
