stm32lx: do not attempt mass-erase in-place of first bank erase

Commit 832f0a5b 'stm32: add mass erase support for STM32L' added
use of mass-erase in-place of bank-erase. This is triggered if first
bank is requested to be fully erased.

This erroneous action completely fails on STM32L162VEY (has 512 KiB
flash in two 256 KiB banks) and also unintently destroying contents of
EEPROM and second flash bank.

Change-Id: I0f13f7b0346747a09c755d72b5b95775ceff5a6f
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@haltian.com>
Reviewed-on: http://openocd.zylin.com/2441


Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: jenkins