flash/nor/efm32: Support flash size smaller than 32k

flash/nor/efm32: Support flash size smaller than 32k

The current implementation fails on devices with less than 32k of
flash (such as several devices in the Zero Gecko family) because
the 'assert' assumes (incorrectly) that the number of flash banks
will always be >= 32.

This change ensures that at least one word of lock bits is always read
in order to support devices with less than 32k of flash.

Signed-off-by: Kevlar Harness <software@klystron.com>
Change-Id: I59febe2cb690c893a5057a5f72918e146cf2afe4
Reviewed-on: http://openocd.zylin.com/3806
Tested-by: jenkins
Reviewed-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>