dts-subscription.sh: shellcheck fixes

[m-iwanicki]

Fixed most info warnings from shellcheck inside dts-subscription.sh script. As we are only using warning and error in our pre-commit a lot of fairly basic problems might go unchecked.
As we are using [ ] instead of [[ ]] at least in most places then we should always double quote variables if we don't want empty variables to error out e.g. as in: Dasharo/dasharo-issues#1156 (comment)
Left couple [ $? -eq ...] as is:

$ shellcheck include/dts-subscription.sh

In include/dts-subscription.sh line 151:
  if [ $? -ne 0 ]; then
       ^-- SC2181 (style): Check exit code directly with e.g. 'if ! mycmd;', not indirectly with $?.


In include/dts-subscription.sh line 180:
  [ $? -ne 0 ] && return 1
    ^-- SC2181 (style): Check exit code directly with e.g. 'if ! mycmd;', not indirectly with $?.


In include/dts-subscription.sh line 201:
  if [ $? -ne 0 ]; then
       ^-- SC2181 (style): Check exit code directly with e.g. 'if ! mycmd;', not indirectly with $?.

For more information:
  https://www.shellcheck.net/wiki/SC2181 -- Check exit code directly with e.g...