jtag: Use 'unsigned int' for 'scan_field.num_bits'

jtag: Use 'unsigned int' for 'scan_field.num_bits'

This patch modifies as little code as possible in order to simplify the
review. Data types that are affected by these changes will be addresses
in following patches.

While at it, apply coding style fixes if these are not too extensive.

Change-Id: Idcbbbbbea2705512201eb326c3e6cef110dbc674
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/8413
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>