flash: reduce code duplication in stm32 flash probe

flash: reduce code duplication in stm32 flash probe

Remove a lot of the repetitive code in stm32f1x flash probe by converting
the large if-selector to a switch, moving the common checks outside it and
concentrating the failure handling to a single point.

Do the same with stm32f2x and stm32lx for consistency.

Change-Id: Ic0ecfb1533c49f5d2108cda5fd20c8372d7c71ef
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/746
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>