mlxsw: spectrum_acl_tcam: Fix incorrect list API usage

mlxsw: spectrum_acl_tcam: Fix incorrect list API usage

Both the function that migrates all the chunks within a region and the
function that migrates all the entries within a chunk call
list_first_entry() on the respective lists without checking that the
lists are not empty. This is incorrect usage of the API, which leads to
the following warning [1].

Fix by returning if the lists are empty as there is nothing to migrate
in this case.

[1]
WARNING: CPU: 0 PID: 6437 at drivers/net/ethernet/mellanox/mlxsw/spectrum_acl_tcam.c:1266 mlxsw_sp_acl_tcam_vchunk_migrate_all+0x1f1/0>
Modules linked in:
CPU: 0 PID: 6437 Comm: kworker/0:37 Not tainted 6.9.0-rc3-custom-00883-g94a65f079ef6 #39
Hardware name: Mellanox Technologies Ltd. MSN3700/VMOD0005, BIOS 5.11 01/06/2019
Workqueue: mlxsw_core mlxsw_sp_acl_tcam_vregion_rehash_work
RIP: 0010:mlxsw_sp_acl_tcam_vchunk_migrate_all+0x1f1/0x2c0
[...]
Call Trace:
 <TASK>
 mlxsw_sp_acl_tcam_vregion_rehash_work+0x6c/0x4a0
 process_one_work+0x151/0x370
 worker_thread+0x2cb/0x3e0
 kthread+0xd0/0x100
 ret_from_fork+0x34/0x50
 ret_from_fork_asm+0x1a/0x30
 </TASK>

Fixes: 6f9579d4e302 ("mlxsw: spectrum_acl: Remember where to continue rehash migration")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Alexander Zubkov <green@qrator.net>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/4628e9a22d1d84818e28310abbbc498e7bc31bc9.1713797103.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>