board: nios2: Check if flash is configured before calling early_flash_cmd_reset()

If CONFIG_CFI_FLASH_MTD is not defined, then we shouldn't perform the
flash early reset.

This commit fixes the following build error:

  nios2-generic.c: In function `__early_flash_cmd_reset':
  nios2-generic.c:23: error: `AMD_CMD_RESET' undeclared (first use in this function)
  nios2-generic.c:23: error: (Each undeclared identifier is reported only once
  nios2-generic.c:23: error: for each function it appears in.)
  nios2-generic.c:24: error: `FLASH_CMD_RESET' undeclared (first use in this function)

which was introduced by:

  commit a113fb39
  Author: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
  Date:   Fri Dec 20 18:34:53 2013 -0300

  board: nios2: Add CONFIG_CFI_FLASH_MTD guard to flash.h header include

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>

Cc: Thomas Chou <thomas@wytron.com.tw>
Reported-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>