drivers/jlink: fix calculate_swo_prescaler formula

drivers/jlink: fix calculate_swo_prescaler formula

a) TPIU_ACPR is defined as:
	SWO_baudrate = TRACECLKIN/(TPIU_ACPR +1)

b) TPIU_ACPR is set by armv7m_trace_tpiu_config()
	target_write_u32(target, TPIU_ACPR, Prescaler-1), so
	TPIU_ACPR = Prescaler-1

Replacing TPIU_ACPR in a), we get:
	SWO_baudrate = TRACECLKIN/Prescaler, so

c)	Prescaler = TRACECLKIN/SWO_baudrate

The Prescaler calculated by calculate_swo_prescaler() is greater by 1:
	Prescaler = TRACECLKIN/SWO_baudrate + 1

The second problem is that even in situations when
an exact baudrate match is possible,
the resulting TRACECLKIN/Prescaler already has a 3% deviation.

For example, TRACECLKIN=88000000, SWO_baudrate=500000,
calculate_swo_prescaler will return Prescaler=171.
The correct value should be Prescaler=176 (TPIU_ACPR=175).

Might be related to https://sourceforge.net/p/openocd/tickets/263/

Change-Id: Ib4d6df6e34685a9be4c2995cb500b2411c76e39b
Signed-off-by: Adrian Negreanu <adrian.negreanu@nxp.com>
Reviewed-on: http://openocd.zylin.com/5807
Tested-by: jenkins
Reviewed-by: Marc Schink <dev@zapb.de>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>