net: tcp: Store return value of net_buf_frags_len() on a size_t

The net_buf_frags_len() function returns a size_t, which is an unsigned
integer.  Store on an appropriate size_t variable instead.

Change-Id: I98aa4c0ddd7c464737436aa9ce13bdc86c11da2b
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>