target/armv7m: cosmetic refactorization

Introduce a variable 'size' and reduce the number of dereferencing
*reg_list_size by using the variable.

Change-Id: I3bdf1485a4ed8e34435e8acb3efd0df8d802508c
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5326
Tested-by: jenkins
Reviewed-by: Christopher Head <chead@zaber.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>