cmd_flash.c: fix fix compile error for boards with DataFlash

Commit 5669ed45 ("cmd_flash.c: fix warning: unused variable
'addr_first'/'addr_last'") changed the #ifdef logic areound the
declaration of these variables and missed a combination of settings
of HAS_DATAFLASH with SYS_NO_FLASH; this patch fixes this.

Also spotted by Alessandro Rubini <rubini@gnudd.com>

Signed-off-by: Wolfgang Denk <wd@denx.de>