target/cortex_m: minor refactoring in cortex_m_store_core_reg_u32()

target/cortex_m: minor refactoring in cortex_m_store_core_reg_u32()

Unlike cortex_m_load_core_reg_u32() storing core register uses
the same code pattern around DHCSR read as offered by the convenience
helper cortex_m_read_dhcsr_atomic_sticky().

Use the helper.

Change-Id: Ia947204944a8b549f3c2be7fb2f717aad18970c4
SeeAlso: 65d762918328 (cortex_m: poll S_REGRDY on register r/w)
SeeAlso: 0dcf95c7171b (target/cortex_m: cumulate DHCSR sticky bits)
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6767
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>