i2c: st: Use HZ_PER_GHZ constant instead of plain number

i2c: st: Use HZ_PER_GHZ constant instead of plain number

Use defined constant to avoid the possible mistakes and to provide
an additional information on the units.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://lore.kernel.org/r/20260112134900.4142954-7-andriy.shevchenko@linux.intel.com