env: common: Factor out the common env_valid check

The check for gd->env_valid is used in both the 'if' and 'else' part of
env_get_char(). Move it into that function instead for simplicity. Drop
that code from the two leaf functions.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>