target/cortex_m: remove fp_code_available counting

fp_code_available looses sync with the real number of free comparators
as soon as cortex_m_set_breakpoint() returns an error. Remove the counter
and always go through the fp_comparator_list to find a free one.

Change-Id: I9f6e06c36d8a57ad11df5155e8a1a3aff6d833a5
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4870
Tested-by: jenkins
Reviewed-by: Jānis Skujenieks <janis.skujenieks@gmail.com>
Reviewed-by: Christopher Head <chead@zaber.com>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>