jtag/drivers/bcm2835gpio: use rounding in delay math

jtag/drivers/bcm2835gpio: use rounding in delay math

After setting adapter speed to some values, the driver
shows the real speed little bit higher.
Although it does not impose a problem from technical point
of view because the difference is smaller than usual speed error,
it looks at least strange to the user. The documentation reads
that real frequency should be same or lower than requested.

Use proper rounding in speed -> delay and delay -> speed
conversions.

Change-Id: I1831112cc58681875548d2aeb688391fb79fa37f
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/7261
Tested-by: jenkins
Reviewed-by: Jonathan Bell <jonathan@raspberrypi.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>