jtag/core: get rid of variable 'jtag'

jtag/core: get rid of variable 'jtag'

The variable 'jtag' is set to 'adapter_driver' during adapter
initialization and is used:
- to check if adapter has been initialized;
- as local copy of adapter_driver.

Introduce a static flag to check if the adapter has been already
initialized and a convenience test function.
Use the test function and the original value of adapter_driver in
the code and drop the variable 'jtag'.

Change-Id: I1b1c54d3b36d7b60390985d787c8449432788141
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6639
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>