[R] address some lintr warnings

[jameslamb]

Contributes to #8012 .

Resolves the following R linting warnings.

style: [infix_spaces_linter] Put spaces around all infix operators.
style: [spaces_left_parentheses_linter] Place a space before left parenthesis, except in a function call.
style: [commas_linter] Commas should always have a space after.
style: [semicolon] Trailing semicolons are not needed.
style: [trailing_whitespace_linter] Trailing whitespace is superfluous.

How I tested this

Ran the linter with the following code.

Rscript tests/ci_build/lint_r.R $(pwd)

[trivialfis]

Thank you for the nice work! We are very close to addressing all the linter warnings!