i2c: altera: Fix potential integer overflow

Factor out 100 from the equation and do 32-bit arithmetic (3 * clk_mhz / 10)
instead of 64-bit.

Notice that clk_mhz is MHz, so the multiplication will never wrap 32 bits
and there is no need for div_u64().

Addresses-Coverity: 1458369 ("Unintentional integer overflow")
Fixes: 0560ad57

 ("i2c: altera: Add Altera I2C Controller driver")
Suggested-by: David Laight <David.Laight@ACULAB.COM>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Thor Thayer <thor.thayer@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>