u-boot: remove driver lookup loop from env_save()

When called with ENVOP_SAVE, env_get_location() only returns the
gd->env_load_location variable without actually checking for
the environment location and priority.

This behaviour causes env_save() to fall into an infinite loop when
the low-level drv->save() call fails.

The env_save() function should not loop through the environment
location list but it should save the environment into the location
stored in gd->env_load_location by the last env_load() call.

Signed-off-by: Nicholas Faustini <nicholas.faustini@azcomtech.com>
Reviewed-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>