drivers: slip: Consistently log pkt/buf allocation failures

There are 3 cases of possible allocation failures, only 1 of them
was logged. Now, all the cases are logged: 1) failure to allocate
net_pkt; 2) failure to allocate very first net_buf for it; 3)
failure to allocate additional net_buf for it (this latter was
the only one logged previously).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>