serial: sc16is7xx: set safe default SPI clock frequency

serial: sc16is7xx: set safe default SPI clock frequency

15 MHz is supported only by 76x variants.

If the SPI clock frequency is not specified, use a safe default clock value
of 4 MHz that is supported by all variants.

Also use HZ_PER_MHZ macro to improve readability.

Fixes: 2c837a8a8f9f ("sc16is7xx: spi interface is added")
Cc:  <stable@vger.kernel.org>
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://lore.kernel.org/r/20231221231823.2327894-4-hugo@hugovil.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>