net: lib: http: ensure SYS_FOREVER_MS behavior

The http_client_req() function's timeout parameter is allowed
to be SYS_FOREVER_MS. However, K_MSEC() does not convert this
to a proper k_timeout_t, so sys_timepoint_calc() ends up
returning 0, which is causes immediate timeouts.

Check for this case specifically and force value passed to
sys_timepoint_calc() to be correct.

Signed-off-by: Pete Skeggs <peter.skeggs@nordicsemi.no>