tty: n_gsm: Improve debug output

Use appropriate print helpers for debug messages.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Link: https://lore.kernel.org/r/20200512115323.1447922-2-gregory.clement@bootlin.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>