subsys/fs: nvs Coverty CID205795 & CID205803 corrections

In nvs writing addresses are u32_t. Coverty reports two situations
where the address could be converted (unwanted) to a signed value.
Both have been corrected.

There is however a general problem with flash API where the addresses
are defined as off_t which is a s32_t. These are converted in the flash
hal to u32_t. As a result of this only half of the possible range can
be used.

Solves #20867 and #20866

Signed-off-by: Laczen JMS <laczenjms@gmail.com>