slip: fix MTU comparation operation when reallocating buffers

In sl_realloc_bufs() there is no reason to check if the requested MTU greater
than or equal to the current MTU value as this function called only
when requested
MTU not equals to the current value. So, the ">=" operation can be
safely replaced
with the ">".

Signed-off-by: Matvejchikov Ilya <matvejchikov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>