realtek: pending upstream rtl9300 i2c speed patch

realtek: pending upstream rtl9300 i2c speed patch

Some 10G optics showed random "module transmit fault indicated" due to I2C
read errors on ONTi ONT-S508CL-8S/XikeStor SKS8300-8X switches. The same
modules work with the original firmware and on other Linux based devices.

There seems to be some differences in how we talk to those modules using
I2C in OpenWRT. To fix this this patch adds support for 50kHz I2C speed on
SFPs and enables that for XikeStor/Onti devices. Since SFPs only transmit
very few bytes this should not have any real downsides.

This patch adds support in the i2c driver for 50kHz and 2.5MHz. In a
second PR I will configure 50kHz in the DTS for the affected devices.

Signed-off-by: Jan Kantert <jan-openwrt@kantert.net>
Link: https://github.com/openwrt/openwrt/pull/22209
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>