SNB is wider than three bits on STM32F4

The SNB part in the FLASH_CR 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>