Instead of checking for all possible constants that every
kernel header with devlink support should have (and defining
HAVE_DECL_DEVLINK as 1 if any of them is present due to the
way AC_CHECK_DECLS works), only check for DEVLINK_CMD_ESWITCH_GET.

This is the name of the constant since kernel 4.11. Between 4.8
and 4.11, the now deprecated spelling DEVLINK_CMD_ESWITCH_MODE_GET
was used.

Assume DEVLINK_ESWITCH_MODE_SWITCHDEV is available, since it was
introduced along with the deprecated spelling.