stm32lx.c: Read IDcode at appropriate address.

stm32lx.c: Read IDcode at appropriate address.

Trying to read the L0 idcode at the L1 idcode address 0xE0042000 often
resulted in an uncatched error. Reading at the right L0 address 0x40015800
afterwards results in reading 0. So access to the device is denied..

Change-Id: I6de92cf99a5d5d46c72f9ba055613cbc5753a951
Signed-off-by: Uwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
Reviewed-on: http://openocd.zylin.com/3883
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>