drivers/jlink: fix (again) calculate_swo_prescaler formula

drivers/jlink: fix (again) calculate_swo_prescaler formula

The prescaler computation should round at the nearest integer
value, not to the next integer value.

Change-Id: I957e0774421211f3c4ba4b312738b1c67b87c4a2
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: 051e80812b1b ("drivers/jlink: fix calculate_swo_prescaler formula")
Reviewed-on: http://openocd.zylin.com/5846
Tested-by: jenkins
Reviewed-by: Adrian M Negreanu <adrian.negreanu@nxp.com>