Staging: et131x: Fix Smatch Warning of Buffer Overflow

This patch fixes the following Sparse Warning in et131x.c-

drivers/staging/et131x/et131x.c:2957 nic_send_packet() error: buffer overflow 'frags' 17 <= 21
drivers/staging/et131x/et131x.c:2959 nic_send_packet() warn: buffer overflow 'frags' 17 <= 21
drivers/staging/et131x/et131x.c:2961 nic_send_packet() error: buffer overflow 'frags' 17 <= 21

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>