jtag/drivers cmsis-dap: do not limit speed so strictly

Adapter clock frequency is set by 32-bit number and most adapters
limit the highest speed safely. There is no reason to impose strict
limit of 5000 kHz if some adapters can do more.

While on it give informative error message in case of zero adapter_khz.

Change-Id: I45c9804678e24496ea769ea9ca6036701b04dde9
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/3945


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>