yaffs: replace IS_ERR with IS_ERR_OR_NULL to check both ERR and NULL

When run below command to mount a yaffs2 FS to nor flash, there
is kernel panic as below:
Unable to handle kernel NULL pointer dereference at virtual address 0000000000000020
......
Internal error: Oops: 96000004 [#1] PREEMPT SMP
Modules linked in:
CPU: 1 PID: 335 Comm: mount Not tainted 5.10.73-yocto-standard #1
......
Call trace:
 yaffs_internal_read_super.constprop.0+0x24c/0x6fc
 yaffs2_internal_read_super_mtd+0x28/0x40
 mount_bdev+0x1cc/0x200
 ......
 el0_sync_handler+0x1a4/0x1b0
 el0_sync+0x180/0x1c0
Because in function yaffs_get_mtd_device(), it check the type of
mtd device. If it is not NAND flash, NULL will be returned. This
causes kernel panic if the ret value is not checked whether it
is NULL or not. So, replace IS_ERR with IS_ERR_OR_NULL to check
both ERR and NULL.

Signed-off-by: Meng Li <Meng.Li@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>