shell: fix margin handling after newlines in formatted help text

Lines following newlines in shell help text were not properly indented.
For example, in "abc def\ndef ghi\njkl mno", the lines "def ghi" and
"jkl mno" would appear without the expected left margin.

The issue was that after processing a newline and setting proper cursor
position, the function would fall through to z_shell_raw_fprintf() which

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>