net: zperf: Fix TOS option not working in zperf

When the zperf command is called with '-S' option which means IP_TOS
for IPv4 and IPV6_TCLASS for IPv6, an error is printed and the
setting does not work. The socket option handling was changed by
commit 77e522a5('net: context: Refactor option setters'), but the
callers of option setters were not changed. This causes the IP_TOS
or IPV6_TCLASS option failed to set. The fix is to use uint8_t to
store the value of the -S option.

Signed-off-by: Maochen Wang <maochen.wang@nxp.com>
(cherry picked from commit e7444dcf)