target/cortex_m: use cortex_m->dcb_dhcsr in cortex_m_soft_reset_halt()

target/cortex_m: use cortex_m->dcb_dhcsr in cortex_m_soft_reset_halt()

cortex_m->dcb_dhcsr caches status of DHCSR register.
Use it instead of local variable in cortex_m_soft_reset_halt()
like in other code.

Extracted from [1].

[1] Antonio Borneo: 6207: cortex_m: rework handling of dcb_dhcsr

Link: https://review.openocd.org/c/openocd/+/6207
Change-Id: I9a0aeba0b6b0b4969f05f4a32fc2fc8d244f56ca
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6677
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>