x86/early_printk: Replace obsolete simple_strtoul() usage with kstrtoint()

Change early_serial_init() to call kstrtoul() instead of calling
obsoleted simple_strtoul().

Signed-off-by: Shuah Khan <shuahkhan@gmail.com>
Cc: Joe Perches <joe@perches.com>
Link: http://lkml.kernel.org/r/1338424803.3569.5.camel@lorien2


Signed-off-by: Ingo Molnar <mingo@kernel.org>