mvtwsi: Fix clock programming

The TWSI_FREQUENCY macro was wrong in 2 ways:
1) It was casting the result of the calculations to an u8, while i2c clk
rates are often >= 100Khz which won't fit in a u8, drop the cast.
2) It had an extra factor of 2 in the divider which neither the datasheet nor
the Linux driver have.

The comment for the default value was wrongly saying that m lives in
bits 4-7, while in reality it is in bits 3-6, as can be seen from the correct
shift by 3 used in i2c_init().

While at it remove the unused twsi_actual_speed variable.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>