Fix unintended sign extension (SIGN_EXTENSION).
The result of "coap_block_size_to_bytes() * (iter - 1)"
extending the sign type.
Issue is the conversion of iter from u8_t to int because it's
an unsigned operand in an binary subtraction with 1 which is
a signed operand with higher conversion rank (int).
Fixes#20880Fixes#20881Fixes#20882Fixes#20883
Coverity CID :205780
Coverity CID :205786
Coverity CID :205806
Coverity CID :205808
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>