SNB is wider than three bits on STM32F4

The SNB part of the FLASH_CR register is four bits wide on F4 devices
and five bits wide on F4_HD devices. F4 devices have a reserved bit
alongside the sector number, so just increase the bitmask to cover all
five bits.

Currently flashing breaks, if flahing sector eight and above before
flashing lower sectors, because the highest bit is never unset.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Merge pull request #277 from swegener/for-upstream

SNB is wider than three bits on STM32F4