selftests: drivers: hw: cleanup shellcheck warnings in the rmon test

selftests: drivers: hw: cleanup shellcheck warnings in the rmon test

If run on the ethtool_rmon.sh script, shellcheck generates a bunch of
false positive errors. Suppress those checks that generate them.

Also cleanup the remaining warnings by using double quoting around the
used variables.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20260330152933.2195885-5-ioana.ciornei@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>