[LIBS] netstacK: Fix ARP entry updation in lwIP

The etharp_find_entry() returns s8_t value. In case of failure,
the returned value will be less than zero. Now, all signed constants
in GCC are treated as s32_t so, when etharp_find_entry() fails at
that time comparison in etharp_update_arp_entry() to check return
value also fails.

For e.g. if etharp_find_entry() returns ERR_MEM=-1 to
etharp_update_arp_entry() then the condition "if (i < 0)" fails
because in GCC i (s8_t) is upgraded to s32_t and then compared
which zero (i.e. we have 255 < 0).

Please note the above described issue in etharp_update_arp_entry()
only see when lwIP is compiled using GCC (which is always the case
for us) hence this patch to fix it.

Signed-off-by: Anup Patel <anup@brainfault.org>