armv7m: Do not ignore 'value' parameter in armv7m_write_core_reg

armv7m: Do not ignore 'value' parameter in armv7m_write_core_reg

Ignoring the value parameter in that function makes its code rather
misleading. Also the only caller of it, armv7m_restore_context already
does the whole "buf_get_u32" conversion business, so using
'value' also removes the waste of doing the conversion twice.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Change-Id: I515979c314d9b59ee1065c55b5bb5747c7e93f01
Reviewed-on: http://openocd.zylin.com/2057
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>