selftests: drv-net: Set shell=True for sysfs writes in devlink_rate_tc_bw.py

selftests: drv-net: Set shell=True for sysfs writes in devlink_rate_tc_bw.py

Commit 7c32f7a2d3db ("selftests: net: py: don't default to shell=True")
changed the cmd() helper to avoid spawning a shell unless explicitly
requested.

The devlink_rate_tc_bw test enables SR-IOV by writing to the
sriov_numvfs sysfs attribute using redirection. Without shell=True the
redirection is not interpreted and the VF device never appears,
causing the test to fail.

Fix by explicitly passing shell=True in the two places that update
sriov_numvfs.

Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Nimrod Oren <noren@nvidia.com>
Link: https://patch.msgid.link/20251130091938.4109055-5-cjubran@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>