vgacon: Use pr_warn instead of pr_warning

vgacon: Use pr_warn instead of pr_warning

As said in commit f2c2cbcc35d4 ("powerpc: Use pr_warn instead of
pr_warning"), removing pr_warning so all logging messages use a
consistent <prefix>_warn style. Let's do it.

Link: http://lkml.kernel.org/r/20191018031850.48498-24-wangkefeng.wang@huawei.com
To: linux-kernel@vger.kernel.org
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: linux-fbdev@vger.kernel.org
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>