[SPARK-9510] [SPARKR] Remaining SparkR style fixes

With the change in this patch, I get no more warnings from `./dev/lint-r` in my machine

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #7834 from shivaram/sparkr-style-fixes and squashes the following commits:

716cd8e [Shivaram Venkataraman] Remaining SparkR style fixes