net: coap: Fix underlying type for block number

The block number in block1/2 options can be encoded on up to 20 bits
according to RFC 7959, therefore the underlying type used in helper
functions to retrieve the block number should be large enough to hold
the result. Therefore, replace the container for block number with
uint32_t instead of uint8_t.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>