tty/console: fix warnings in drivers/tty/serial/kgdboc.c

The con_debug_leave/con_debug_enter functions are stubbed out
by defining them to (0), which causes harmless build warnings.
Using proper inline functions is the normal way to deal with
this.

Without this patch, building the ARM bcm2835_defconfig results in:

drivers/tty/serial/kgdboc.c: In function 'kgdboc_pre_exp_handler':
drivers/tty/serial/kgdboc.c:279:3: warning: statement with no effect [-Wunused-value]
drivers/tty/serial/kgdboc.c: In function 'kgdboc_post_exp_handler':
drivers/tty/serial/kgdboc.c:293:3: warning: statement with no effect [-Wunused-value]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Anton Vorontsov <anton.vorontsov@linaro.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>