bnxt_en: fix memory leak in bnxt_nvm_test()

Free the kzalloc'ed buffer before returning in the success path.

Fixes: 5b6ff128

 ("bnxt_en: implement callbacks for devlink selftests")
Signed-off-by: Vikas Gupta <vikas.gupta@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/1666020742-25834-1-git-send-email-michael.chan@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>