stm32f1x: improve variable naming and avoid potential divide-by-zero

stm32f1x: improve variable naming and avoid potential divide-by-zero

Don't call a variable num_pages if it holds the flash size. Also rearrange
flash size to num_pages calculation to avoid divide-by-zero if there will
be a device with < 1024 byte pages someday.

Change-Id: I2febea39694a2f9750de141f52ec88ae1599c086
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/211
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>