target/armv7m: fix register number in armv7m_get_core_reg()

target/armv7m: fix register number in armv7m_get_core_reg()

armv7m_get_core_reg() calls arm->read_core_reg()
arm->read_core_reg() expects the register number as an index
to core reglist, not an ARMv7M specific register code.
Use reg->number instead of armv7m_reg->num.

The change solves assert
  src/target/armv7m.c:222: armv7m_read_core_reg: Assertion
  `num < (int)armv7m->arm.core_cache->num_regs' failed.
when gdb 'info reg' is issued on a Cortex-M target and
no cortex_m_debug_entry() has been called since OpenOCD start
(target was halted before OpenOCD start).

Change-Id: I32a2294693ef979b613be93aeceb3b0eb06ee6df
Ticket: https://sourceforge.net/p/openocd/tickets/216/
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5203
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>