net: lib: lwm2m: fix formula for expected next block

Block size is a 3-bit value mapping values [0, 6] to powers of 2 in the
range [16, 1024]. Value 7 is invalid.
The previous formula was not working in case the client's preferred size
was 4 (or more) times bigger than the server's.
This commit takes into account also the case the client's preferred size
is smaller than the server's.

Signed-off-by: Marco Argiolas <marco.argiolas@ftpsolutions.com.au>