net: ethernet: rtsn: Fix a null pointer dereference in rtsn_probe()

Add check for the return value of rcar_gen4_ptp_alloc()
to prevent potential null pointer dereference.

Fixes: b0d3969d ("net: ethernet: rtsn: Add support for Renesas Ethernet-TSN")
Cc: stable@vger.kernel.org
Signed-off-by: Haoxiang Li <haoxiang_li2024@163.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://patch.msgid.link/20250703100109.2541018-1-haoxiang_li2024@163.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>