shell: devmem: address `cmd_dump` multiple call problem

Call `getopt_init()` to reset `state->optind` instead of directly setting
`optind = 1`. The `getopt()` function uses `getopt_state` from
`getopt_state_get` for state handling, so `getopt_init()` should be called
to correctly run the command again.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>