bootutil: fix FIH int conversion for security_cnt

Currently there's a compile error when building MCUboot
with HW_ROLLBACK_PROT due to a comparison
when decoding the security_cnt fih_int value. In the security_cnt.h
it is stated that this value must be between 0 and UINT32_MAX
so this cast would not cause any undefined behaviour.

Signed-off-by: Roland Mikhel <roland.mikhel@arm.com>
Change-Id: Iee158a31955ff43b73e67a0c08e7a086077b9eb5