env: suppress a spurious warning with GCC 7.1

env: suppress a spurious warning with GCC 7.1

GCC 7.1 seems to be smart enough to track val through the various
static inline functions, but not smart enough to see that val will
always be initialised when no error is returned.  This triggers
the following warning:
  env/mmc.c: In function 'mmc_get_env_addr':
  env/mmc.c:121:12: warning: 'val' may be used uninitialized in this function [-Wmaybe-uninitialized]

To make it easier for compiler to understand what is going on, let's
initialise val.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>