CVE-2025-68220: net: ethernet: ti: netcp: Standardize knav_dma_open_channel to return NULL on error

In the Linux kernel, the following vulnerability has been resolved:

net: ethernet: ti: netcp: Standardize knavdmaopenchannel to return NULL on error

Make knavdmaopenchannel consistently return NULL on error instead of ERRPTR. Currently the header include/linux/soc/ti/knavdma.h returns NULL when the driver is disabled, but the driver implementation does not even return NULL or ERRPTR on failure, causing inconsistency in the users. This results in a crash in netcpfreenavigatorresources as followed (trimmed):

Unhandled fault: alignment exception (0x221) at 0xfffffff2 [fffffff2] pgd=80000800207003, pmd=82ffda003, pte=00000000 Internal error: : 221 [#1] SMP ARM Modules linked in: CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.17.0-rc7 #1 NONE Hardware name: Keystone PC is at knavdmaclosechannel+0x30/0x19c LR is at netcpfreenavigatorresources+0x2c/0x28c

[... TRIM...]

Call trace: knavdmaclosechannel from netcpfreenavigatorresources+0x2c/0x28c netcpfreenavigatorresources from netcpndoopen+0x430/0x46c netcpndoopen from devopen+0x114/0x29c devopen from devchangeflags+0x190/0x208 devchangeflags from netifchangeflags+0x1c/0x58 netifchangeflags from devchangeflags+0x38/0xa0 devchangeflags from ipautoconfig+0x2c4/0x11f0 ipautoconfig from dooneinitcall+0x58/0x200 dooneinitcall from kernelinitfreeable+0x1cc/0x238 kernelinitfreeable from kernelinit+0x1c/0x12c kernelinit from retfromfork+0x14/0x38 [... TRIM...]

Standardize the error handling by making the function return NULL on all error conditions. The API is used in just the netcpcore.c so the impact is limited.

Note, this change, in effect reverts commit 5b6cb43b4d62 ("net: ethernet: ti: netcpcore: return error while dma channel open issue"), but provides a less error prone implementation.