cmd: nvedit: Mark a few functions static

cmd: nvedit: Mark a few functions static

Some functions are not used anywhere except the same file
where they are defined. Mark them static. This helps avoiding
the compiler warnings:

  cmd/nvedit.c:201:5: warning: no previous prototype for ‘do_env_ask’ [-Wmissing-prototypes]
  cmd/nvedit.c:315:5: warning: no previous prototype for ‘do_env_callback’ [-Wmissing-prototypes]
  cmd/nvedit.c:384:5: warning: no previous prototype for ‘do_env_flags’ [-Wmissing-prototype ]

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>