use correct clock to communicate with PMW3360 sensor

[koron]

It is too fast to communicate with PMW3360DM sensor, 70MHz.
(It is PMW3360DM internal clock, is not for SPI)

PMW3360DM's datasheet recommends 2MHz for SPI.

QMK treats 70MHz as zero divisor and ATmega32U4 works with 4MHz.
It works but twice of the recommended speed.

This PR fix it, to use 2MHz clock to communicate with PMW3360DM.

[Yowkees]

Thank you very much for your reviewing of PMW3360's datasheet.
This chage must make Keyball much better. Thank you.