malta: disable unnecessary built-in CDROM support

malta: disable unnecessary built-in CDROM support

The CDROM is not needed for booting and can be included by selecting the
loadable module as a package instead.

This also avoids triggering a memory allocation failure during probing of
the CDROM due to lack of low 16MB DMA memory, as decribed in FS#3278:
https://bugs.openwrt.org/index.php?do=details&task_id=3278

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>
Link: https://github.com/openwrt/openwrt/pull/3289
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>