devlink: stop using NL_SET_ERR_MSG_MOD

NL_SET_ERR_MSG_MOD inserts the KBUILD_MODNAME and a ':' before the actual
extended error message. The devlink feature hasn't been able to be compiled
as a module since commit f4b6bcc7

 ("net: devlink: turn devlink into a
built-in").

Stop using NL_SET_ERR_MSG_MOD, and just use the base NL_SET_ERR_MSG. This
aligns the extended error messages better with the NL_SET_ERR_MSG_ATTR
messages as well.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>