The driver uses the string "slcan" to populate
tty_ldisc_ops::name. KBUILD_MODNAME also evaluates to "slcan". Use
KBUILD_MODNAME to get rid on the hardcoded string names.

Similarly, the pr_info() and pr_err() hardcoded the "slcan"
prefix. Define pr_fmt so that the "slcan" prefix gets automatically
added.

CC: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Link: https://lore.kernel.org/all/20220728070254.267974-2-dario.binacchi@amarulasolutions.comSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>