mmc: sdhci: fix "pwr may be used uninitialized" warning

This patch fixes a warning when compiling the sdhci driver:
  pwr may be used uninitialized in sdhci_set_power

Tested with the following compiler versions: 4.2.4 and 4.4.4

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Chris Ball <cjb@laptop.org>