drivers: slip: Rework logging to follow conventions

Previously, there was boolean CONFIG_SLIP_DEBUG, which effectively
switched between "logging off" and "debug-level logging". Instead,
switch to CONFIG_SYS_LOG_SLIP_LEVEL (the naming of the option follows
existing conventions) which allows to select any of the standard 5
logging levels.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>