ALSA: core: timer: Use NSEC_PER_SEC macro

1000000000L is number of ns per second, use NSEC_PER_SEC macro to replace
it to make it more readable

Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Link: https://patch.msgid.link/20240902071622.3519787-1-ruanjinjie@huawei.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>