esp: Fix error handling on layer 2 xmit.

esp_output_tail() and esp6_output_tail() can return negative
and positive error values. We currently treat only negative
values as errors, fix this to treat both cases as error.

Fixes: fca11ebd ("esp4: Reorganize esp_output")
Fixes: 383d0350

 ("esp6: Reorganize esp_output")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>