fw_setenv: Unbreak fw_setenv caused by buggy MEMISLOCKED use

Commit "fw_setenv: lock the flash only if it was locked before"
checks for Locked status with uninitialized erase data.
Address by moving the test for MEMISLOCKED.

Fixes: 8a726b852502 ("fw_setenv: lock the flash only if it was locked before")
Signed-off-by: Joakim Tjernlund <joakim.tjernlund@infinera.com>