flash: stm32l handle flash size read failure

Rather than failing if the flash size reg cannot be read lets assume
we have the max flash size fitted.

It is quite common on early ST silicon to not correct support this register.

Change-Id: Ife058d60ae0027faad2c929ebd5b7fe2ef27234d
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/743


Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>