west: runners: esp32: respect --esp-device during erase

When --erase was specified, esp32 runner was autodetecting serial port to
be used, regardless of --esp-device argument.

Append '--port SERIAL_DEVICE' parameter earlier, so that erase command
invocation uses explicitly specified serial device.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>