cmd: eficonfig: adjust struct eficonfig_entry, field key

cmd: eficonfig: adjust struct eficonfig_entry, field key

The number EFICONFIG_ENTRY_NUM_MAX - 1 must be printable
to field key of struct eficonfig_entry.

Fixes: 1f0583beeb32 ("eficonfig: set EFICONFIG_ENTRY_NUM_MAX to INT_MAX - 1")
Fixes: 23aa0502a4b9 ("eficonfig: increase the number of menu entries")
Addresses-Coverity-ID: 583414 - Out-of-bounds write
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>